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

docs/architecture/websocket.md

@@ -285,7 +285,15 @@ semantically-validating hook dials/prepares the backend before the
 reply, and a bounded failure resolves `channel:open_failed` with
 `details.reason` (the channel never exists consumer-side). The
 per-registration timeout override bounds the establisher when the
-dispatch carries no deadline. The `OpenableAlpns` request-extension
+dispatch carries no deadline. The establisher's `Establishment.plan`
+payload (alkcall 0.6.0 / ADR-049 amendment 2 — a typed-opaque
+`ChannelPlan`) is threaded process-locally to the pump handler's
+second parameter: the establisher and the handler agree on the
+concrete type, so ALPN crates dial/allocate in the establisher and
+deliver the live handle to their handler without a side-channel
+handoff. Data-plane handlers that pump a channel stream against a
+peer's split halves can use alkcall's `channels::pump_bidi` helper
+(alkcall 0.6.0 / ADR-050). The `OpenableAlpns` request-extension
 fallback is available for bare-registry/custom
 upgrade routes. Cap policy is the `ChannelsPolicy` extension (one
 instance consulted by both the open wrappers and the demux teardown).