refactor(gateway): migrate to alkcall 0.2 promoted gateway module

Bump the alkcall dependency to 0.2 (with the gateway feature) and
converge on the promoted shared pieces:

- The local dispatch spine (gateway/dispatch.rs, 721 lines) is deleted;
  GatewayDispatch, schema_disclosure_denial, and DEFAULT_DEADLINE are
  re-exported from alkcall::gateway (alkcall ADR-048). The 30 s default
  deadline preserves the previous behavior exactly.
- gateway/schema_cache.rs (PublishSchemaCache) is deleted: alkcall CF-003
  compiles publish_schema at registration time and exposes
  OperationRegistry::publish_validator; the /publish chunk stream
  resolves against it. Un-compilable schemas are now rejected at
  registration, so the two end-to-end fail-closed tests were reworked
  into a registration-rejection test (a stronger guarantee).
- schema_disclosure_denial consumers (to_mcp, routes) use alkcall's
  promoted implementation; the alkhttp-local copy is gone (ADR-071
  updated: the guard stays as defense-in-depth, the implementation no
  longer forks).
- CF-001: from_wss drop monitor and the WS overlay tests use
  CallError::connection_closed; the review-001-ws-eof-signal race tests
  now assert retryable CONNECTION_CLOSED on both resolution paths (the
  tolerated non-retryable INTERNAL write-failure outcome is gone).
- Added CHANGELOG.md (Keep a Changelog), Unreleased section records the
  bump and convergence.

Verification: cargo test default 453 ok, wss 470 ok, mcp 526 ok,
all-features 575 ok; clippy -D warnings clean (default + all-features,
all-targets); fmt clean; cargo doc warning-free.

Net: -1093 lines.

tasks/websocket/review-001-ws-eof-signal.md

@@ -84,11 +84,11 @@ and `held_session_drop_during_call_registration…` cover the CON-02 race
 asserts the sweep resolves it retryable; the original
 `connection_drop_fails_in_flight_calls_retryable_no_hang` stays green.
 One tolerated outcome note: the held-session race can also resolve via
-alkcall's write-failure path (`failed to write request frame`,
-`INTERNAL` — the mux dies between registration and write), which is
-prompt-but-not-retryable; that path is accepted in the race tests (the
-retryable assertion lives with the sweep and pre-drop tests where the
-call is guaranteed in-flight).
+alkcall's write-failure path — since alkcall CF-001 landed
+(`CallError::connection_closed`), that path now resolves
+retryable `CONNECTION_CLOSED` too, so the race tests assert
+retryable-only (updated in the alkcall-0.2 bump; the retryable
+assertion no longer needs the sweep-test carve-out).
 
 Verification: cargo test 219 ok; cargo test --features wss 231 ok (3x
 flake check); cargo clippy --all-targets -- -D warnings (default;