chore(deps): consume alkcall 0.6.0 — OpenHandler gains the plan parameter

Mechanical pass for the alkcall 0.6.0 bump (review 007's
establishment follow-ups sweep: R-01 Establishment plan payload,
R-02 OpenHandler lifetime doc note, R-03 pump_bidi extraction):

- bump alkcall 0.5 -> 0.6 (gateway feature unchanged)
- the OpenHandler signature gained a plan parameter —
  Fn(Value, Option<ChannelPlan>, Connection, AuthContext) ->
  JoinHandle<()> — so the test's echo_open_handler closure gains
  `_plan` (the only closure-construction site in this crate)
- no ferry change: this crate constructs no Establishment and passes
  the Option<OpenEstablisher> through unchanged (OpenableAlpn fields
  and register_openable_with_establisher threading are unchanged in
  0.6.0); the establisher's Establishment.plan now reaches the pump
  handler's second parameter process-locally (ADR-049 amendment 2)
- docs updated: websocket.md (plan threading + pump_bidi pointer),
  OpenableAlpn field docs, with_ws_openable_alpns doc comment
- CHANGELOG [Unreleased] consumption-wave entry

Verification: cargo test (454 passed), cargo test --all-features
(587 passed), clippy -D warnings (default + all-features),
fmt --check, cargo doc --no-deps clean

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 = "0.5", features = ["gateway"] }
+alkcall = { version = "0.6", features = ["gateway"] }
 arc-swap = { version = "1", optional = true }
 axum = { version = "0.8", optional = true, features = ["ws"] }
 bytes = "1"