refactor: drop alktype dependency; add BAST doc for chunk header

- Remove alktype from Cargo.toml (its only usage was a thin wrapper
  over jsonschema::options().build())
- Replace alktype::validation::build_validator with direct jsonschema
  in dispatch.rs
- Add docs/architecture/chunk-header.bast.json — the chunk header's
  BAST (Binary Abstract Syntax Tree) machine-readable wire spec
- Embed as channels::wire::CHUNK_HEADER_BAST via include_str! so
  downstream Rust crates can consume it without a file lookup
- Add test asserting the embedded BAST doc is valid JSON and matches
  the wire format
- Update AGENTS.md §10 and implementation-specialist.md: BAST docs
  are the contract; trivial/hot-path formats stay hand-rolled,
  complex formats use the alktype engine or codegen

Verification:
- cargo test: 543 passed, 0 failed
- cargo clippy --all-targets -- -D warnings: clean
- cargo fmt --check: clean
- cargo doc --no-deps: clean
- BAST doc compiles + round-trips against alktype v0.2.0 engine

docs/architecture/README.md

@@ -272,8 +272,8 @@ feature flags) or in the downstream alknet crate.
 - `@alkdev/alknet: docs/architecture/` — the source architecture docs
   these were ported from (renumbered from alknet ADR-001..094 to alkcall
   ADR-001..045)
-- `@alkdev/alktype` — the binary struct engine, used for channels chunk
-  header layout and JSON payload schema validation
+- `@alkdev/alktype` — the binary struct engine; compiles BAST documents
+  (e.g. `chunk-header.bast.json`) into readers/writers/validators
 - `@alkdev/pubsub` — the TypeScript EventEnvelope prior art the call
   wire format was derived from
 

docs/architecture/channels-wire.md

@@ -21,6 +21,14 @@ multiplexing on the `BiStream` the channels layer gives it.
 
 8 bytes of header, followed by `length` bytes of opaque payload.
 
+The machine-readable spec is the BAST document
+[`chunk-header.bast.json`](chunk-header.bast.json) (embedded in the
+crate as `channels::wire::CHUNK_HEADER_BAST`). BAST is plain JSON —
+any language can consume it; the `alktype` crate compiles it into
+readers/writers/validators, and future codegen derives
+language-specific implementations. The Rust implementation is
+hand-rolled (the hot path); the BAST document is the contract.
+
 | field | offset | width | meaning |
 |-------|--------|-------|---------|
 | `channel_id` | 0 | 4 (BE) | The logical channel this chunk belongs to. Channel 0 is pre-negotiated as `alk/call` (ADR-036). Channels 1..N are opened dynamically via `channel/open` (ADR-037). |

docs/architecture/chunk-header.bast.json

@@ -0,0 +1,12 @@
+{
+  "$defs": {
+    "ChunkHeader": {
+      "kind": "struct",
+      "endian": "big",
+      "fields": [
+        { "name": "channel_id", "kind": "uint32" },
+        { "name": "length", "kind": "uint32" }
+      ]
+    }
+  }
+}

docs/reviews/003-alpn-prefix-rename.md

@@ -355,7 +355,11 @@ These were checked and are correct — the rename does not affect them:
 - **No logic changes:** the rename is purely mechanical. No function
   signatures, trait bounds, or control flow change. ✓
 - **No new dependencies, no feature flag changes.** ✓
-- **`alktype` dependency unaffected.** ✓
+- **`alktype` dependency:** dropped in a follow-up commit (the only
+  usage, `alktype::validation::build_validator`, was a thin wrapper
+  over `jsonschema`; the chunk header BAST document
+  `docs/architecture/chunk-header.bast.json` carries the wire-format
+  spec without the dependency). ✓
 - **483 tests pass before the rename; they should all pass after.** ✓
 - **The `alknet/` prefix in `derive_alpn_from_op_name` is the only
   non-mechanical change** — the function's logic stays the same, only