phase 4: architecture docs + BAST schema + renumbered ADRs

Port the alknet-tty architecture docs into alktty and add the BAST
document for the alk/tty wire format. Docs-only; no Rust source
changes.

Spec docs (docs/architecture/, flat layout — single-crate repo):
- overview.md — crate purpose, two-carriage model, deps, ALPN,
  backend location map, feature gates
- tty-wire.md — 5-byte chunk codec, control channel split
  (STREAM_CTRL_IN=3 / STREAM_CTRL_OUT=4), sentinels
- tty-backend.md — TtyBackend trait, TtyHandle, TtyControl,
  REQ-TTY-01 (backends need not be natively async)
- tty-adapter.md — TtyAdapter, three-pump driver, exit-chunk
  ordering (ADR-004), cancel cleanup (ADR-005), access control
- tty-local.md — LocalTtyBackend (local feature module), PTY +
  pipe modes, REQ-TTY-02 (signal forwarding to process group)
- README.md — architecture index

ADRs (docs/architecture/decisions/, renumbered 001..008 from
alknet 052,053,054,055,056,057,077,093 in order):
- 001 wire format + two-carriage model (incl. Phase 7 control-
  channel split amendment)
- 002 TtyBackend trait + TtyHandle
- 003 local backend placement (records both the alknet sibling-
  crate decision and the alktty single-crate consolidation behind
  a local feature)
- 004 exit code on a control chunk
- 005 backend cleanup on session cancel
- 006 self-contained negotiation framing
- 007 tty inside channels (reversed by 008; kept for historical
  context with reversal notice)
- 008 channels pure channel multiplexing (reverses 007; TTY
  always uses its 5-byte format)

BAST document (docs/architecture/tty-bast.md):
- Normative JSON spec for the alk/tty wire format, conforming to
  the BAST meta-schema at https://alk.dev/bast/v1/schema
- 5-byte chunk header (struct, big-endian: stream_type uint8,
  length uint32) + StreamType enum (Stdin=0..CtrlOut=4)
- ControlMessage union (field-name discriminator on type:
  resize/signal/eof/exit) with documented deviation that on-wire
  control payloads are UTF-8 JSON, not BAST's binary union
  encoding
- NegotiationFrame (4-byte BE length + UTF-8 JSON body) +
  NegotiateRequest / TerminalParams JSON shapes
- StreamType enum deviation noted: on-wire uint8, not BAST's
  standard u32 enum index (chunk header is 5 bytes, not 8)
- alktty does not depend on alktype; the hand-rolled wire.rs is
  the runtime codec, the BAST is the human-readable contract

AGENTS.md: fixed the ADR mapping table to match the plan's 8-to-8
mapping (the previous table substituted ADR-050 for 054, relabeled
056 as control-message split, dropped 077, and added a new
control-split ADR at 006 — inconsistent with both the plan and the
prose). ADR-050 (dynamic resource ownership) is an alkcall/alknet-
core ADR, not tty-specific, and is not ported; the Phase 7 control
split stays as an amendment inside ADR-001, mirroring alknet.

Verification (all pass, no Rust source changed):
- cargo test (80 passed)
- cargo test --all-features (103 passed)
- cargo clippy --all-targets -- -D warnings (clean)
- cargo fmt --check (clean)
- cargo check --target wasm32-unknown-unknown (clean)
- cargo clippy --target wasm32-unknown-unknown -- -D warnings
  (clean)
- cargo doc --no-deps: 9 pre-existing intra-doc-link warnings in
  src/session.rs and src/channels.rs (untouched by this commit;
  not introduced here)
- BAST JSON parses; StreamType indices match wire.rs constants
  (0=Stdin..4=CtrlOut)
- all markdown cross-reference links resolve

docs/plans/project-setup.md

@@ -1,6 +1,8 @@
 # Project Setup Plan
 
-Status: draft (revised 2026-08-17 to reflect landed upstream changes)
+Status: draft (revised 2026-08-17 to reflect landed upstream changes;
+Phase 4 landed 2026-08-17 — architecture docs + BAST schema +
+renumbered ADRs)
 Last updated: 2026-08-17
 
 ## Overview
@@ -415,32 +417,76 @@ operation spec — the registry runs the ACL before the wrapper, so the
    `local` feature in `src/lib.rs`:
    `#[cfg(feature = "local")] pub mod local;`
 
-### Phase 4: Architecture docs + BAST schema
+### Phase 4: Architecture docs + BAST schema — landed 2026-08-17
 
-1. Port the 5 spec docs from `alknet/docs/architecture/crates/tty/`
+1. Ported 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:
+   `overview.md` (the crate overview; the alknet `README.md` index is
+   ported as `docs/architecture/README.md`), `tty-wire.md`,
+   `tty-backend.md`, `tty-adapter.md`, `tty-local.md`. Each is renamed
+   `alknet-tty` → `alktty`, `alknet/tty` → `alk/tty`, `alknet-core` →
+   `alkcall::core`, `alknet-call` → `alkcall`, `alknet-tty-local` →
+   alktty's `local` feature module, and cross-references to alknet
+   ADRs are renumbered to alktty's ADR range (001..008).
+2. Ported the 8 alknet ADRs (052, 053, 054, 055, 056, 057, 077, 093)
+   into `docs/architecture/decisions/` renumbered 001..008 in order:
+   - [001](../architecture/decisions/001-wire-format-and-two-carriage.md)
+     ← alknet ADR-052 — wire format + two-carriage model (incl. the
+     Phase 7 control-channel split amendment)
+   - [002](../architecture/decisions/002-ttybackend-trait-and-ttyhandle.md)
+     ← alknet ADR-053 — `TtyBackend` trait + `TtyHandle`
+   - [003](../architecture/decisions/003-local-backend-placement.md)
+     ← alknet ADR-054 — local backend placement (records both the
+     alknet sibling-crate decision and the alktty single-crate
+     consolidation behind a `local` feature)
+   - [004](../architecture/decisions/004-exit-code-on-control-chunk.md)
+     ← alknet ADR-055 — exit code on a control chunk
+   - [005](../architecture/decisions/005-backend-cleanup-on-session-cancel.md)
+     ← alknet ADR-056 — backend cleanup on session cancel
+   - [006](../architecture/decisions/006-negotiation-framing-self-contained.md)
+     ← alknet ADR-057 — self-contained negotiation framing
+   - [007](../architecture/decisions/007-tty-inside-channels.md)
+     ← alknet ADR-077 — TTY inside channels (reversed by 008; kept for
+     historical context with its reversal notice pointing to 008)
+   - [008](../architecture/decisions/008-channels-pure-channel-multiplexing.md)
+     ← alknet ADR-093 — channels pure channel multiplexing (reverses
+     007; TTY always uses its 5-byte format)
+3. Wrote
+   [`docs/architecture/tty-bast.md`](../architecture/tty-bast.md) —
+   the BAST (Binary Abstract Syntax Tree) document for the `alk/tty`
+   wire format. Conforms to the BAST meta-schema at
+   `https://alk.dev/bast/v1/schema`; validatable by any JSON Schema
+   Draft 2020-12 validator. 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`,
+   - The five 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`).
+     `resize`, `signal`, `eof`, `exit`) with the documented deviation
+     that on-wire control payloads are UTF-8 JSON, not BAST's binary
+     union encoding.
    - 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.
+   - The `StreamType` enum's documented deviation: on-wire encoding is
+     `uint8` (1 byte), not BAST's standard `u32` enum index (4 bytes)
+     — the chunk header is 5 bytes, not 8.
+4. Wrote [`docs/architecture/README.md`](../architecture/README.md) —
+   the architecture index: documents table, ADR table (with alknet
+   origin numbers and port status), key design principles, open
+   questions, and references.
+
+The ADR mapping follows the plan's 8-to-8 list (ADR-052→001, 053→002,
+054→003, 055→004, 056→005, 057→006, 077→007, 093→008). ADR-050
+(dynamic resource ownership) is an alkcall/alknet-core ADR, not
+tty-specific, and is not ported into alktty's ADR range; the
+access-control work that declares against the ADR-050 model is
+described in `tty-adapter.md` and the ADR-001/002 ported docs, which
+reference ADR-050 by its alknet number. The Phase 7 control-channel
+split (`STREAM_CTRL_IN` = 3, `STREAM_CTRL_OUT` = 4) is an amendment
+inside ADR-001, mirroring alknet (it was not a standalone ADR there
+either). `AGENTS.md` was updated to match this mapping.
 
 ### Phase 5: Tests