chore(deps): consume alkcall 0.7.0 — CONNECTION_CLOSED maps 503

Dependency wave + one wire-surface behavior fix:

- bump alkcall 0.6 -> 0.7 (gateway feature unchanged). No code change
  forced: this crate constructs no ServingConfig (the one 0.7.0
  struct-literal break — from_wss keeps the pure-consumer
  from_connection; opt-in serving remains the review-006 Part C
  deferred decision), and the CF-006 per-call opener-identity overlay
  is behavior-only upstream (hub-forwarded WS-ferry opens now resolve
  the end client's identity, the intended effect)
- gateway error table maps CONNECTION_CLOSED -> 503 Service
  Unavailable: alkcall 0.7.0's CF-007 amendment makes it the eighth
  protocol code (ADR-016's list is formal at eight); it previously
  fell to the catch-all 500, misreporting a retryable,
  provably-undelivered call as a permanent server failure. retryable:
  true, so the retry_after details path applies on 503. Tests pin the
  mapping and the Retry-After passthrough
- ADR-023 amended: the protocol-code list is eight (ALREADY_EXISTS +
  CONNECTION_CLOSED rows added to the §3 table, normative rule and
  call.error code list updated); http-adapters.md six-code references
  de-staled

Verification: cargo test (456 passed), cargo test --all-features
(589 passed), clippy -D warnings (default + all-features), fmt
--check, cargo doc --no-deps (0 warnings)

Cargo.toml

@@ -42,7 +42,7 @@ h2 = ["server", "dep:hyper", "hyper-util/http2", "hyper/http2"]
 http1 = ["server", "dep:hyper", "hyper-util/http1", "hyper/http1"]
 
 [dependencies]
-alkcall = { version = "0.6", features = ["gateway"] }
+alkcall = { version = "0.7", features = ["gateway"] }
 arc-swap = { version = "1", optional = true }
 axum = { version = "0.8", optional = true, features = ["ws"] }
 bytes = "1"