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/overview.md

@@ -246,10 +246,14 @@ PTY allocation code. See [ADR-003](decisions/003-local-backend-placement.md).
   / `STREAM_CTRL_OUT` halves, JSON control messages), sentinels, and
   the fixed-channel-set rationale.
 - **[tty-bast.md](tty-bast.md)** — 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`, validatable by any JSON Schema
-  Draft 2020-12 validator.
+  Tree) document for the binary framing layer of the `alk/tty` wire
+  format (the 5-byte chunk header and the negotiation frame's 4-byte
+  length prefix); a normative JSON spec conforming to the BAST
+  meta-schema at `https://alk.dev/bast/v1/schema`, validatable by any
+  JSON Schema Draft 2020-12 validator. The JSON payloads
+  (`NegotiateRequest`, `ControlMessage`, `TerminalParams`) are
+  specified in `tty-wire.md` and the Rust source, not in the BAST —
+  BAST describes binary layouts, not JSON shapes.
 - **[tty-backend.md](tty-backend.md)** — the `TtyBackend` trait,
   `TtyParams`, `TtyHandle`, `TtyControl`. The inversion point between
   the wire-format adapter and the backends. Carries REQ-TTY-01