alktty/docs/plans/project-setup.md

# Project Setup Plan

Status: draft (revised 2026-08-17 to reflect landed upstream changes)
Last updated: 2026-08-17

## Overview

Extract `alknet-tty` + `alknet-tty-local` from the alknet mono-repo into a
standalone `alktty` crate — a producer/consumer protocol crate on top of
alkcall channels. This is the first protocol crate built on alkcall and
establishes patterns that `alktunnels` and others will follow.

The local backend (`alknet-tty-local`) is folded into `alktty` as a
feature-gated `local` submodule rather than kept as a sibling crate.
Both the original `alknet-tty` (the wire/adapter/backend-trait half) and
`alknet-tty-local` (the `portable_pty` + `tokio::process` half) are
ported together, so the single crate carries the full stack: the
`alk/tty` protocol + the one backend implementation that ships with it.
A docker or SSH backend would be a separate crate; the local backend is
small and the cyclic-dependency workaround it required in the alknet
mono-repo (see ADR-054) doesn't apply to a single crate with a feature
flag.

## Upstream changes that landed since the first draft

These were "open questions" or "upstream changes needed" in the
2026-08-14 draft and are now resolved. The plan reflects the as-built
shape, not the original intent.

### alkcall 0.1.1 — ALPN rename done

- `CHANNELS_ALPN`: `b"alknet/channels"` → `b"alk/channels"` (in
  `alkcall/src/channels/adapter.rs`).
- Test at `channels/client.rs:767` uses `"alk/tty"` (was
  `"alknet/tty"`).
- Semver showed no public API change → alkcall bumped 0.1.0 → 0.1.1.
- `alkcall` `Cargo.toml` dependency needs updating from `0.1.0` to
  `0.1.1` (the scaffold still pins `0.1.0`).

### alkcall — `Identity.resources` shape

The alknet-core `Identity` had `resources: HashMap<String, String>` (a
flat resource-type → resource-id map). alkcall's `Identity` (in
`alkcall::core::auth`) has `resources: HashMap<String, Vec<String>>` —
one resource type can carry multiple resource ids. The ported adapter
tests that build an `Identity` by hand need this shape; the production
code only reads `identity.scopes` (the scope-gate at negotiation), so
the change is test-side.

### alkcall — `OwnershipProvider` / `OwnershipStore` signature

- `OwnershipProvider::owns(&self, identity, resource_type,
  resource_id, action: &str)` — added an `action` parameter (was 3
  args, now 4). The adapter's call site
  (`provider.owns(id_ref, kind, &id, "tty")`) already passes an
  `"tty"` action string, which is the new arity. No change to the
  production adapter; the mock and test fixtures need the 4-arg shape.
- `OwnershipStore::record(&self, identity, resource_type,
  resource_id)` — **dropped** the old `action` parameter that
  alknet-core's variant took. The adapter test's
  `store.record(&owner, "container", "c1").await` matches the new
  arity. (The scaffold's plan previously listed an `OwnershipStore`
  import that no longer needs an action arg.)

### alkcall — `AuthContext::anonymous(alpn)`

alkcall added a convenience constructor `AuthContext::anonymous(alpn)`
that builds an `AuthContext` with no identity, no fingerprint, no
remote address — only the ALPN. Useful in adapter tests; not used by
the production adapter (which receives the `AuthContext` from the
dispatch path).

### alktype 0.2.0 — BAST format pivot

alktype was reworked: the v0.1.0 `AlkType:*` custom-keyword JSON Schema
backend was dropped in favor of a straightforward `kind`-based BAST
(Binary Abstract Syntax Tree) format inspired by unist ASTs. A BAST
document is a plain JSON file (conforming to a JSON Schema meta-schema)
that describes binary layouts using `kind` strings (`"uint8"`,
`"uint32"`, `"struct"`, etc.) and `$defs`/`$ref` for composition.

`alktty` will **not** depend on alktype — the hand-rolled
`ChunkReader`/`ChunkWriter` in `wire.rs` already works and is
straightforward. The BAST format matters here only as a normative
**schema document** for the `alk/tty` wire format: a JSON file under
`docs/architecture/` that downstream consumers (and alktype, if
desired) can consume to generate validators, layout maps, or readers
in any language. The schema is a documentation artifact, not a runtime
dependency.

## Crate Structure

Single crate (`alktty`) with the local backend gated behind a `local`
feature. This is a "tty-specific monorepo" — one crate, one concern,
both halves of the original alknet split (`alknet-tty` +
`alknet-tty-local`) folded in.

```
alktty/
├── src/
│   ├── lib.rs              # crate root, re-exports
│   ├── wire.rs             # ChunkReader/ChunkWriter (5-byte format)
│   ├── negotiation.rs      # NegotiateRequest, framing reader/writer
│   ├── control.rs          # ControlMessage enum, signal_from_name
│   ├── backend.rs          # TtyBackend trait, TtyHandle, TtyParams, TtyError
│   ├── adapter.rs          # TtyAdapter (ProtocolHandler), drive_session
│   ├── session.rs          # TtySession — typed consumer client
│   ├── channels.rs         # OpenHandler factory, register_openable helper
│   └── local/              # (behind `local` feature — the folded alknet-tty-local)
│       ├── mod.rs          # LocalTtyBackend re-export
│       ├── backend.rs      # LocalTtyBackend (dispatches terminal: Some→pty, None→pipe)
│       ├── pty.rs          # PTY mode (portable_pty, 3 std threads → tokio channels, #[cfg(unix)])
│       └── pipe.rs         # Pipe/runner mode (tokio::process::Command, cross-platform)
├── docs/
│   ├── architecture/
│   │   ├── README.md       # architecture index
│   │   ├── tty-wire.md     # wire format spec (5-byte chunk + 4-byte neg frame)
│   │   ├── tty-bast.md     # BAST JSON document for the alk/tty wire format
│   │   ├── tty-backend.md  # TtyBackend trait + LocalTtyBackend
│   │   ├── tty-adapter.md  # TtyAdapter + drive_session
│   │   ├── tty-local.md    # PTY + pipe modes, REQ-TTY-01/02
│   │   └── decisions/      # renumbered ADRs
│   └── plans/
│       └── project-setup.md  # this document
├── Cargo.toml
├── LICENSE-APACHE
└── LICENSE-MIT
```

## Key Design Decisions

### 1. Single crate, feature-gated local backend

The old split (`alknet-tty` + `alknet-tty-local`) had a cyclic dependency
problem: `alknet-tty-local` depends on `alknet-tty` for the trait, and
ADR-054 wanted `alknet-tty` to re-export `LocalTtyBackend` behind a
`local` feature — which cargo rejects. The assembly-layer workaround
(consumer depends on both crates directly) works but is awkward.

Single crate with `local` feature solves this cleanly. The
`portable-pty` and `tokio-util` deps are optional. A docker-only
deployment doesn't pull in PTY code.

**Folded-in scope.** The single crate carries both halves of the
original alknet split: the wire/adapter/backend-trait half (from
`alknet-tty`) and the local backend (from `alknet-tty-local`). The
local backend is small (~1.4k lines), tightly coupled to the trait,
and the cyclic-dep workaround it required in the alknet mono-repo
doesn't exist in a single crate. A docker or SSH backend would still
be a separate crate — those have real external deps (`bollard`,
`russh`) and their own resource models.

### 2. Dependency: `alknet-core` → `alkcall::core`

All types previously from `alknet-core` now come from `alkcall::core`:
- `ProtocolHandler`, `Connection`, `BiStream`, `BidiStreamSource`
- `AuthContext`, `Identity`, `IdentityProvider`
- `AccessControl`, `OwnershipProvider`, `OwnershipStore`,
  `InMemoryOwnershipStore`
- `HandlerError`, `StreamError`

The `TtyAdapter`'s `impl ProtocolHandler` changes which crate the trait
comes from. The trait shape is identical. The `Identity` struct's
`resources` field is `HashMap<String, Vec<String>>` in alkcall (was
`HashMap<String, String>` in alknet-core); the production adapter only
reads `identity.scopes`, so the change is test-side.

`Cargo.toml` pins `alkcall = "0.1.1"` (the scaffold's `0.1.0` pin is
stale — 0.1.1 is the ALPN-rename release; same public API per semver).

### 3. Channels integration (the new part)

This is where alktty becomes a proper producer/consumer on alkcall.

**ALPN strings (settled).** `alk/tty` (direct) and `alk/channels`
(multiplexed). The `alknet/<name>` → `alk/<name>` rename landed in
alkcall 0.1.1 — `CHANNELS_ALPN` is `b"alk/channels"`, the channels
client test uses `"alk/tty"`. No upstream work remains; we just use
the new strings.

**Producer half** — two paths to expose TTY:

- **Direct ALPN** (`alk/tty`): `TtyAdapter` implements `ProtocolHandler`.
  Existing code, just import migration. Used for direct-connect scenarios
  (browser terminal over WebTransport, standalone TTY endpoint).

- **Through channels** (`alk/channels`): Register an `OpenHandler` via
  `ChannelCore::register_openable()`. The handler receives a `Connection`
  (the channel's data plane with ALPN `alk/tty`), calls `accept_bi()`,
  and runs the same `drive_session` on the resulting `BiStream`.

  Per ADR-093 (reversal of ADR-077): TTY always uses its 5-byte format.
  The channels layer strips its 8-byte header and hands TTY the payload
  transparently. The same `wire.rs` code runs in both modes — only the
  `BiStream` source differs.

  The channels integration code is small: a `register_openable` helper
  function that takes a `ChannelCore`, `OperationRegistry`, and backend
  map, and registers the per-ALPN open op (`channels/tty/sub` —
  consumer-opens). This lives in `src/channels.rs`. The op spec carries
  the `channel_open` marker (`ChannelOpenSpec::new("alk/tty")`) and the
  `AccessControl` for the scope-gate + ownership check; the channels
  wrapper does `check_open` → `open_channel` → spawn the `OpenHandler`
  → respond with `{ channel_id }`.

**Consumer half** — `TtySession`:

A typed client that wraps the wire protocol. Two constructors:
- `TtySession::connect_direct(connection)` — for direct `alk/tty`
- `TtySession::open_via_channels(client, params)` — opens a channel via
  `ChannelClient`, negotiates, returns a session handle

The session handle exposes:
- `send_stdin(bytes)` / `close_stdin()`
- `recv_stdout()` / `recv_stderr()` — streams
- `resize(cols, rows)`, `signal(name)`
- `wait()` — awaits exit code

### 4. Negotiation framing — keep self-contained

ADR-057 says the negotiation framing is self-contained (~30 lines, format
coincides with alkcall's by convention). Even though alkcall is now a
dependency, we keep the self-contained framing because:

- The payload is `NegotiateRequest`, not `EventEnvelope` — alkcall's
  `FrameFramedReader` is hardcoded to deserialize `EventEnvelope`
- The framing is trivial (4-byte BE length prefix + body)
- Avoids coupling to alkcall's internal wire types for a TTY-specific payload

### 5. Producer/consumer framing

The architecture README in alkcall describes protocol crates as having two
halves:

1. **Producer half** — `register_*()` functions that take an
   `&mut OperationRegistry` and register ops with handlers. For channels,
   an `OpenHandler` factory.

2. **Consumer half** — a typed client wrapper around `CallConnection` or
   `ChannelClient` that exposes the crate's ops as async methods.

We make this explicit in the module structure:
- `src/adapter.rs` + `src/channels.rs` = producer half
- `src/session.rs` = consumer half

### 6. Access control and auth

The old code had minimal access control (a scope-gate at negotiation).
With alkcall, we get:

- **Operation-level ACL**: `AccessControl` on `OperationSpec` — scopes,
  resource ownership checks (`required_scopes`, `required_scopes_any`,
  `resource_type` + `resource_id_path`, `resource_action`)
- **Channel lifecycle policy**: `ChannelLifecyclePolicy` — per-identity
  channel caps, open/close hooks (consulted by the `ChannelCore`
  wrapper, not by alktty directly)
- **Ownership**: `OwnershipProvider` — runtime-spawned resource tracking
  (terminal sessions are resources per ADR-050). alkcall's
  `OwnershipProvider::owns` takes an `action: &str` arg (the adapter
  passes `"tty"`); `OwnershipStore::record` takes no action arg.

### 7. BAST schema as a documentation artifact

The `alk/tty` wire format gets a BAST (Binary Abstract Syntax Tree)
document under `docs/architecture/tty-bast.md`. BAST is alktype's
JSON-based binary-layout vocabulary (alktype 0.2.0 dropped the old
`AlkType:*` custom-keyword backend in favor of this `kind`-based
format). The document is a normative spec: downstream consumers (and
alktype, if desired) can consume it to generate validators, layout
maps, or readers in any language with a JSON parser.

**alktty does not depend on alktype.** The hand-rolled
`ChunkReader`/`ChunkWriter` in `wire.rs` is the runtime codec; the BAST
document is the human-readable contract that describes what those
types round-trip. Keeping them as separate artifacts avoids pulling in
alktype as a runtime dep for a codec that's already ~200 lines and
works. If we later want runtime validation against the BAST (e.g., to
reject malformed chunks at the framing boundary with a generated
validator), alktype becomes an optional dep — the BAST document is
already there to feed it.

The `TtyAdapter`'s direct-ALPN path keeps the existing ad-hoc scope
check (`has_scope(identity, TTY_OPEN_SCOPE)`) for the scope gate and
optionally consults `OwnershipProvider` for the resource-ownership
check. The channels path gets both for free via
`ChannelCore::register_openable`, which wires `AccessControl` into the
operation spec — the registry runs the ACL before the wrapper, so the
`OpenHandler` only needs to validate params and spawn the protocol.

## Migration Strategy

### Phase 0: Scaffold hygiene (small fixes before porting)

1. Bump `alkcall` in `Cargo.toml` from `"0.1.0"` → `"0.1.1"` (ALPN
   rename release).
2. Add `futures = "0.3"` to `[dependencies]` (the `BoxFuture` alias
   uses `futures::future::BoxFuture`; alkcall pulls it transitively
   but alktty should declare it directly to not rely on a transitive
   dep for a public type alias).
3. Confirm the `local` feature already wires `portable-pty` +
   `tokio-util` as optional deps (it does in the scaffold). The
   `local` module's `pty.rs` additionally needs `libc` under
   `cfg(unix)` — already in `[target.'cfg(unix)'.dependencies]`.

### Phase 1: Port core types from `alknet-tty` (no new logic)

1. Port `wire.rs` — no `alknet-core` imports; just `std::io`, `tokio::io`,
   `bytes`, `thiserror`. Rename crate references in doc comments
   (`alknet/tty` → `alk/tty`, `alknet-tty` → `alktty`).
2. Port `control.rs` — no `alknet-core` imports; `serde`, `bytes`,
   `libc` (under `cfg(unix)`). Rename `alknet/tty` → `alk/tty` in
   doc comments.
3. Port `negotiation.rs` — no `alknet-core` imports; `std::io`,
   `bytes`, `tokio::io`, `thiserror`, `serde_json`. Rename doc
   references.
4. Port `backend.rs` — no `alknet-core` imports; `std`, `async_trait`,
   `bytes`, `futures_core`, `tokio::sync`, `tokio_stream`. Change the
   `BoxFuture` type alias from `Pin<Box<dyn Future + Send>>` to
   `futures::future::BoxFuture<'static, T>` (functionally identical;
   the alias matches the alkcall convention and lets us drop the
   `Pin`/`Future` imports).
5. Port `adapter.rs` — change imports:
   - `alknet_core::auth::{AuthContext, Identity}` →
     `alkcall::core::{AuthContext, Identity}`
   - `alknet_core::ownership::OwnershipProvider` →
     `alkcall::core::OwnershipProvider`
   - `alknet_core::types::{Connection, HandlerError, ProtocolHandler,
     StreamError}` → `alkcall::core::{Connection, HandlerError,
     ProtocolHandler, StreamError}`
   - Change `b"alknet/tty"` → `b"alk/tty"` in `TtyAdapter::alpn()`.
   - The `handle()` method signature is identical (alkcall's
     `ProtocolHandler` shape matches alknet-core's).
6. Port the unit tests inside each module alongside the production
   code. The adapter tests need:
   - `Identity.resources: HashMap<String, Vec<String>>` (was
     `HashMap<String, String>` in alknet-core).
   - `OwnershipStore::record(&self, identity, resource_type,
     resource_id)` — 3 args, no `action` (alkcall dropped it).
   - `OwnershipProvider::owns(identity, rt, rid, action)` — 4 args
     (alkcall added `action`).

### Phase 2: Channels integration (new code)

1. Create `src/channels.rs`:
   - `register_openable` helper — takes `ChannelCore`, backend map,
     `OperationRegistry`, `AuthContext`; builds the `OperationSpec`
     for `channels/tty/sub` with `ChannelOpenSpec::new("alk/tty")`,
     `AccessControl` carrying `TTY_OPEN_SCOPE` + resource-type/ownership
     if a backend declares a `resource_id`; calls
     `ChannelCore::register_openable(spec, open_handler, registry, auth)`.
   - `TtyOpenHandler` — the `OpenHandler` (an
     `Arc<dyn Fn(Value, Connection, AuthContext) -> JoinHandle<()>`)
     that receives a channel `Connection`, calls `accept_bi()`, runs
     `drive_session` on the resulting `BiStream`.
2. Create `src/session.rs`:
   - `TtySession` struct — typed consumer client
   - `TtySession::connect_direct(connection)` — direct `alk/tty`
   - `TtySession::open_via_channels(client, params)` — opens a channel
     via `ChannelClient::open_channel("channels/tty/sub", params,
     "alk/tty")`, negotiates, returns a session handle
   - Methods: `send_stdin`, `recv_stdout`, `recv_stderr`, `resize`,
     `signal`, `wait`

### Phase 3: Local backend (behind `local` feature) — folded `alknet-tty-local`

1. Create `src/local/mod.rs` (re-exports `LocalTtyBackend`),
   `src/local/backend.rs`, `src/local/pty.rs`, `src/local/pipe.rs`.
2. Port from `alknet-tty-local/src/`:
   - `backend.rs` — change `alknet_tty::backend::{...}` →
     `crate::backend::{...}`. `LocalTtyBackend::allocate` dispatches
     on `params.terminal`: `Some` → `pty::allocate_pty`, `None` →
     `pipe::allocate_pipe`.
   - `pty.rs` — `portable_pty` + 3 std threads (reader/writer/waiter)
     feeding tokio mpsc/oneshot. `#[cfg(unix)]`-only: PTY mode uses
     `libc::kill(-pgid, sig)` for process-group signal forwarding.
     Update `alknet_tty::backend::{BoxFuture, TtyControl, ...}` →
     `crate::backend::{...}` and `alknet_tty::control::signal_from_name`
     → `crate::control::signal_from_name`.
   - `pipe.rs` — `tokio::process::Command` + `tokio_util::io::ReaderStream`
     for stdout/stderr. Cross-platform: signal forwarding uses
     `libc::kill(pid, sig)` under `cfg(unix)`, falls back to
     `Child::kill()` on non-Unix. Update `alknet_tty::...` → `crate::...`.
3. `LocalTtyBackend` implements `crate::backend::TtyBackend`. Wire the
   `local` feature in `src/lib.rs`:
   `#[cfg(feature = "local")] pub mod local;`

### Phase 4: Architecture docs + BAST schema

1. Port the 5 spec docs from `alknet/docs/architecture/crates/tty/`
   into `docs/architecture/` (flat layout, not the `crates/tty/`
   subpath — this is a single-crate repo now):
   `tty-wire.md`, `tty-backend.md`, `tty-adapter.md`, `tty-local.md`,
   plus an `overview.md` index.
2. Port relevant ADRs (052, 053, 054, 055, 056, 057, 077, 093) —
   renumber into alktty's ADR range (001..008), update cross-references
   (`alknet/tty` → `alk/tty`, `alknet-tty-local` → `alktty`'s `local`
   feature, `alknet-core` → `alkcall::core`).
3. Write `docs/architecture/tty-bast.md` — the BAST JSON document for
   the `alk/tty` wire format. Covers:
   - The 5-byte chunk header (`struct` with `endian: "big"`:
     `stream_type: uint8`, `length: uint32`).
   - The four stream-type channels as an `enum` (`Stdin=0`,
     `Stdout=1`, `Stderr=2`, `CtrlIn=3`, `CtrlOut=4`).
   - The control-message `union` (field-name discriminator on `type`:
     `resize`, `signal`, `eof`, `exit`).
   - The negotiation frame as a separate `struct` (4-byte BE length
     prefix + UTF-8 JSON `NegotiateRequest` body) — annotated as
     out-of-band for the chunk codec but documented for completeness.
   - Conforms to the BAST meta-schema at
     `https://alk.dev/bast/v1/schema`; validatable by any JSON Schema
     Draft 2020-12 validator.
4. Write `docs/architecture/README.md` index.

### Phase 5: Tests

1. Port existing unit tests from `alknet-tty` (wire, negotiation,
   control, adapter) — these live inline in each `src/*.rs` module's
   `#[cfg(test)] mod tests`.
2. Port integration tests from `alknet-tty-local/tests/` (negotiation,
   pipe, pty) into `tests/` at the crate root. The pty test stays
   `#[cfg(unix)]`.
3. Add channels integration tests — end-to-end `register_openable` +
   `ChannelClient::open_channel` + `drive_session` round-trip. Use
   the in-memory `MockBackend` from `backend.rs` for the producer
   side so the test doesn't need a real PTY.

## Open Questions

### OQ-1: ALPN strings (RESOLVED — landed in alkcall 0.1.1)

**Decision**: `alk/tty` (direct) and `alk/channels` (multiplexed). The
`alknet/<name>` → `alk/<name>` rename landed in alkcall 0.1.1:
`CHANNELS_ALPN` is `b"alk/channels"`, the channels client test uses
`"alk/tty"`. Semver showed no public API change → 0.1.0 → 0.1.1.

### OQ-2: Channel open operation name (RESOLVED)

The alkcall test uses `channels/tty/sub` as the operation name. The
convention from ADR-047 is `channels/<alpn>/sub` for subscribe (consumer
opens) and `channels/<alpn>/pub` for publish (producer opens). Since TTY
is consumer-opens (the client requests a shell), `channels/tty/sub` is
correct.

**Decision**: `channels/tty/sub` for consumer-opens, `channels/tty/pub`
reserved for future producer-opens use case.

### OQ-3: `TtySession` API — stream vs callback (RESOLVED)

**Decision**: Stream-based (`Stream<Item = Bytes>`). Consistent with the
backend trait's `TtyHandle` shape.

### OQ-4: `drive_session` — keep the three-pump pattern? (RESOLVED)

**Decision**: Keep it. It works, it's tested, and ADR-093 means the same
code runs in both direct and channels modes.

### OQ-5: `BoxFuture` type alias (RESOLVED)

**Decision**: Use `futures::future::BoxFuture<'static, T>`. alkcall
already pulls in `futures`, so no new transitive dep; alktty declares
`futures = "0.3"` directly so the public alias doesn't ride on a
transitive dep.

### OQ-6: BAST schema scope (RESOLVED)

**Decision**: Documentation artifact only — `docs/architecture/tty-bast.md`.
alktty does not depend on alktype. The hand-rolled `wire.rs` codec is
the runtime; the BAST document is the normative contract for downstream
consumers. If runtime validation against the BAST becomes desirable
later, alktype becomes an optional dep and the BAST document is already
there to feed it.

## Upstream Changes Needed in alkcall

All four landed in alkcall 0.1.1 (no public API change per semver).
Nothing is blocking alktty; this section is kept as a historical record
of what was pushed.

1. **ALPN convention**: `alknet/<name>` → `alk/<name>` across docs, ADRs,
   and test code. The test at `channels/client.rs:767` now uses
   `ChannelOpenSpec::new("alk/tty")` (was `"alknet/tty"`).

2. **`CHANNELS_ALPN` constant**: `b"alknet/channels"` → `b"alk/channels"`
   in `alkcall/src/channels/adapter.rs`.

3. **ADR-004** (ALPN convention): Updated from `alknet/` prefix to `alk/`.

4. **Architecture README**: The dependency layering diagram and pattern
   section reference `alk/tty` and `alk/channels`.

## Risks and Concerns

### Risk: alkcall is v0.1.x — API stability

alkcall is published at 0.1.1. Breaking changes are expected at this
major-zero stage. The integration surface we use
(`ChannelCore::register_openable`, `ChannelClient`, `ProtocolHandler`,
`Connection`, `Identity`, `OwnershipProvider`) is the core API and
likely stable, but the channels module in particular may evolve.

**Mitigation**: We own alkcall and can push changes as needed. This is
the first real consumer, so we'll find and fix issues upstream rather
than working around them. Pin `alkcall = "0.1.1"` and bump deliberately.

### Risk: `drive_session` complexity

The three-pump driver is ~1500 lines with edge cases around exit ordering,
cancel cleanup, and bidirectional control. Porting it is mechanical but
needs careful review.

**Mitigation**: The existing code has extensive tests. Port tests first
or alongside.

### Risk: Local backend PTY bridge

The PTY mode uses 3 std threads feeding tokio channels. This is a
well-understood pattern (wezterm uses it) but is inherently platform-
specific (Unix only for PTY, pipe mode works cross-platform).

**Mitigation**: Gate PTY behind `#[cfg(unix)]` as the existing code does.
Pipe mode works on all platforms.

### Risk: BAST schema drift

The BAST document in `docs/architecture/tty-bast.md` is a documentation
artifact; the runtime codec is the hand-rolled `wire.rs`. If the wire
format evolves (new stream_type, control message variant, negotiation
field) and the BAST doc isn't updated, downstream consumers generated
from the BAST will be wrong.

**Mitigation**: Add a test that parses the BAST document with
`serde_json` and asserts the stream-type enum values match
`wire.rs`'s `STREAM_STDIN`/`STREAM_STDOUT`/`STREAM_STDERR`/
`STREAM_CTRL_IN`/`STREAM_CTRL_OUT` constants. Cheap; catches the
common drift case. (Doesn't require alktype as a dep — just
`serde_json`, which we already have.)

## References

- [alkcall architecture README](/workspace/@alkdev/alkcall/docs/architecture/README.md)
- [alkcall channels operations](/workspace/@alkdev/alkcall/src/channels/operations.rs) — `ChannelCore::register_openable`, `OpenHandler`
- [alkcall channels client](/workspace/@alkdev/alkcall/src/channels/client.rs) — `ChannelClient::open_channel`, reference integration pattern
- [alktype BAST format](/workspace/@alkdev/alktype/docs/architecture/bast-format.md) — normative format spec for the `tty-bast.md` document
- [alknet-tty source](/workspace/@alkdev/alknet/crates/alknet-tty/src/) — port origin (wire, control, negotiation, backend, adapter)
- [alknet-tty-local source](/workspace/@alkdev/alknet/crates/alknet-tty-local/src/) — port origin (local backend, pty, pipe)
- [alknet-tty architecture docs](/workspace/@alkdev/alknet/docs/architecture/crates/tty/) — port origin for the spec docs
- [ADR-093](/workspace/@alkdev/alknet/docs/architecture/decisions/093-channels-pure-channel-multiplexing.md) — TTY always uses 5-byte format
- [ADR-077](/workspace/@alkdev/alknet/docs/architecture/decisions/077-tty-inside-channels.md) — reversed, historical context