feat(websocket): ride alkcall 0.5.0 — OpenableAlpn establisher + timeout (ADR-049 Unit 3)

Mechanical pass for the alkcall 0.5.0 bump (review 006 sequencing:
ADR-049 → alkcall 0.5.0 → alkhttp mechanical pass):

- bump alkcall 0.4 → 0.5 (gateway feature unchanged)
- OpenableAlpn gains establisher: Option<OpenEstablisher> and
  establisher_timeout: Option<Duration> (both default None via
  OpenableAlpn::new — existing constructions compile unchanged) plus
  a with_establisher(establisher, timeout) builder
- the install_channel_zero ferry calls
  register_openable_with_establisher, threading both fields through
- docs updated (websocket.md, adapter.rs doc comments)

The alkcall 0.5.0 open_channel error-type change (ADR-049 §4) needs
no change here — this crate never calls open_channel (from_wss uses
ChannelClient::from_connection only).

Verification: cargo test (454 passed), cargo test --all-features
(587 passed), cargo clippy --all-targets/-D warnings (default +
all-features), cargo fmt --check

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 = "0.4", features = ["gateway"] }
+alkcall = { version = "0.5", features = ["gateway"] }
 arc-swap = { version = "1", optional = true }
 axum = { version = "0.8", optional = true, features = ["ws"] }
 bytes = "1"