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

src/channels/wire.rs

@@ -34,6 +34,17 @@ pub const MAX_CHUNK_LEN: u32 = 16 * 1024 * 1024;
 /// without an explicit open op exchange.
 pub const CHANNEL_ID_ZERO: u32 = 0;
 
+/// The chunk header's BAST (Binary Abstract Syntax Tree) document —
+/// the machine-readable wire-format spec. The canonical copy is
+/// `docs/architecture/chunk-header.bast.json`; this const embeds it so
+/// downstream Rust crates can consume it without a file lookup. BAST is
+/// plain JSON consumable by any language; the `alktype` crate compiles
+/// it into readers/writers/validators, and future codegen derives
+/// language-specific implementations from it. The hand-rolled
+/// [`parse_header`]/[`write_header`] functions are the hot path; the
+/// BAST document is the contract.
+pub const CHUNK_HEADER_BAST: &str = include_str!("../../docs/architecture/chunk-header.bast.json");
+
 /// The parsed 8-byte chunk header.
 ///
 /// `length = 0` is the EOF sentinel — the reassembled stream interprets
@@ -251,6 +262,39 @@ mod tests {
         }
     }
 
+    #[test]
+    fn chunk_header_bast_is_valid_json_and_describes_the_wire_format() {
+        let doc: serde_json::Value =
+            serde_json::from_str(CHUNK_HEADER_BAST).expect("BAST doc is valid JSON");
+        let def = doc
+            .get("$defs")
+            .and_then(|d| d.get("ChunkHeader"))
+            .expect("ChunkHeader def present");
+        assert_eq!(def.get("kind").and_then(|v| v.as_str()), Some("struct"));
+        assert_eq!(def.get("endian").and_then(|v| v.as_str()), Some("big"));
+        let fields = def
+            .get("fields")
+            .and_then(|v| v.as_array())
+            .expect("fields");
+        assert_eq!(fields.len(), 2);
+        assert_eq!(
+            fields[0].get("name").and_then(|v| v.as_str()),
+            Some("channel_id")
+        );
+        assert_eq!(
+            fields[0].get("kind").and_then(|v| v.as_str()),
+            Some("uint32")
+        );
+        assert_eq!(
+            fields[1].get("name").and_then(|v| v.as_str()),
+            Some("length")
+        );
+        assert_eq!(
+            fields[1].get("kind").and_then(|v| v.as_str()),
+            Some("uint32")
+        );
+    }
+
     #[tokio::test]
     async fn read_header_round_trips_through_duplex() {
         let (mut reader, mut writer) = tokio::io::duplex(64);