docs: fix tty-bast.md to cover only binary framing, not JSON payloads

The previous BAST document modeled the JSON payloads (NegotiateRequest,
ControlMessage and its resize/signal/eof/exit variants, TerminalParams)
as BAST struct/union definitions with uint16/int32 fields. That was a
category error: BAST describes binary data layouts, and per the BAST
format spec itself, "a BAST document cannot validate a JSON payload."
The control and negotiation payloads on the wire are UTF-8 JSON text
serialized via serde_json, not struct-encoded binary — the uint16/int32
field widths implied a binary encoding that does not exist on the wire
and would have misled any generated validator.

The rewrite keeps only the genuinely-binary framing layer:

- ChunkHeader (5-byte: stream_type u8 + length u32 BE)
- StreamType enum (name->index table; documented deviation: on-wire
  is uint8, not BAST's standard u32 enum index)
- Chunk (header + length-prefixed bytes payload)
- NegotiationFrame (4-byte BE length prefix + UTF-8 JSON body,
  modeled as bytes since the body's JSON interpretation is above the
  BAST layer)

The JSON shapes (NegotiateRequest, ControlMessage, TerminalParams)
remain specified in tty-wire.md and implemented by the Rust source
(src/negotiation.rs, src/control.rs), which are the source of truth
for those payloads. Cross-references in tty-wire.md, overview.md, and
README.md updated to reflect the simplified scope.

The drift-detection test (project plan "Risk: BAST schema drift")
still works unchanged — it asserts the StreamType enum values match
wire.rs's STREAM_* constants, and that enum is retained.

Docs-only change; no Rust source changes.

Verification:
- cargo test --all-features -> 122 tests pass (unchanged)
- cargo clippy --all-targets --all-features -- -D warnings -> clean
- cargo fmt --check -> clean
- cargo doc --no-deps -> no new warnings (9 pre-existing rustdoc link
  warnings in src/, unchanged)
- BAST JSON block parses as valid JSON (4 : ChunkHeader,
  StreamType, Chunk, NegotiationFrame)

docs/architecture/README.md

@@ -11,7 +11,7 @@ Syntax Tree) document for the wire format, and the ADRs.
 |----------|--------|-------------|
 | [overview.md](overview.md) | draft | Crate purpose, the two-carriage model in brief, dependencies, ALPN, backend location map, feature gates |
 | [tty-wire.md](tty-wire.md) | draft | The wire format: negotiation frame (JSON carriage), raw chunk codec (`[stream_type: u8][length: u32 be][payload]`), control channel split into `STREAM_CTRL_IN` / `STREAM_CTRL_OUT` halves, sentinels |
-| [tty-bast.md](tty-bast.md) | draft | The BAST (Binary Abstract Syntax Tree) document for the `alk/tty` wire format; a normative JSON spec conforming to the BAST meta-schema at `https://alk.dev/bast/v1/schema` |
+| [tty-bast.md](tty-bast.md) | draft | The BAST (Binary Abstract Syntax Tree) document for the binary framing layer of the `alk/tty` wire format (5-byte chunk header + negotiation length prefix); a normative JSON spec conforming to the BAST meta-schema at `https://alk.dev/bast/v1/schema`. JSON payloads are specified in `tty-wire.md` and the Rust source, not in the BAST |
 | [tty-backend.md](tty-backend.md) | draft | `TtyBackend` trait, `TtyParams`, `TtyHandle`, `TtyControl` — the inversion point between the wire-format adapter and the backends. Carries REQ-TTY-01 (backends need not be natively async) |
 | [tty-adapter.md](tty-adapter.md) | draft | `TtyAdapter` (`ProtocolHandler` on `alk/tty`): session lifecycle, three-pump bidirectional driver, negotiation errors, exit-chunk ordering (ADR-004), access control, session-cancel cleanup (ADR-005) |
 | [tty-local.md](tty-local.md) | draft | `LocalTtyBackend` (the `local` feature module): `portable_pty` (PTY) and `tokio::process::Command` (pipe/runner). Carries REQ-TTY-02 (signal forwarding to the process group) |