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.

Cargo.toml

@@ -22,12 +22,13 @@ h2 = ["dep:hyper", "hyper-util/http2", "hyper/http2"]
 http1 = ["dep:hyper", "hyper-util/http1", "hyper/http1"]
 
 [dependencies]
-alkcall = "0.1.1"
+alkcall = { version = "0.2", features = ["gateway"] }
 arc-swap = "1"
 axum = { version = "0.8", features = ["ws"] }
 hyper = { version = "1", optional = true, features = ["server"] }
 hyper-util = { version = "0.1", features = ["server", "service", "tokio"] }
 httpdate = "1"
+jsonschema = { version = "0.46", default-features = false }
 reqwest = { version = "0.13", default-features = false, features = ["json", "stream", "rustls"] }
 reqwest-middleware = "0.5"
 reqwest-retry = "0.9"
@@ -45,7 +46,6 @@ http = "1"
 http-body-util = "0.1"
 url = "2"
 percent-encoding = "2"
-jsonschema = { version = "0.46", default-features = false }
 parking_lot = "0.12"
 rmcp = { version = "1.8", optional = true, default-features = false, features = [
     "client",