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.

src/server/state.rs

@@ -48,10 +48,6 @@ pub(crate) struct RouterState {
     pub(crate) ws_session_slots: Arc<tokio::sync::Semaphore>,
     /// Idle-read timeout for the WS pumps (WS-01); `None` disables.
     pub(crate) ws_idle_timeout: Option<std::time::Duration>,
-    /// Compile-once `publish_schema` validator cache for the `/publish`
-    /// route (GW-01 follow-up); built empty at adapter construction and
-    /// populated lazily per operation.
-    pub(crate) publish_schemas: crate::gateway::schema_cache::PublishSchemaCache,
 }
 
 impl axum::extract::FromRef<RouterState> for crate::websocket::SessionState {
@@ -100,7 +96,6 @@ mod tests {
                 crate::websocket::DEFAULT_WS_MAX_SESSIONS,
             )),
             ws_idle_timeout: Some(crate::websocket::DEFAULT_WS_IDLE_TIMEOUT),
-            publish_schemas: crate::gateway::schema_cache::PublishSchemaCache::new(),
         };
         let extracted: DecoyConfig = axum::extract::FromRef::from_ref(&state);
         assert!(matches!(extracted, DecoyConfig::Redirect { .. }));