feat(review 007 Unit 1): Establishment carries the channel plan (R-01) + lifetime doc (R-02)

Implements ADR-049 amendment 2 — the reserved Establishment payload is
filled, and the OpenHandler lifetime contract is documented.

- Establishment { plan: Option<ChannelPlan> } with ChannelPlan =
  Arc<dyn Any + Send + Sync>: typed-opaque, because the payload an
  establisher hands the pump handler is a live handle (dialed socket,
  TTY handle), not JSON — the review's Option<Value> sketch could not
  satisfy its own verification gate. #[non_exhaustive] keeps a future
  carrier change from being another break. Construction:
  Establishment::new(plan) / Establishment::default().
- OpenHandler gains the plan parameter:
  Fn(Value, Option<ChannelPlan>, Connection, AuthContext) ->
  JoinHandle<()>. Separate parameter (not merged into input) — a
  typed payload cannot ride the JSON input; no schema collision.
  Wire surface unchanged: the plan is process-local (establisher ->
  wrapper -> handler).
- run_open_wrapper threads establishment.plan to the handler; None
  when no establisher is registered. Kills the alktunnels-POC
  side-channel handoff (resource-keyed slot + poll loop) whose
  concurrent same-resource race is now unreachable — each open's
  establisher result flows to its own handler.
- Lifetime contract documented (R-02, doc-only half): the returned
  JoinHandle must track the data-plane lifetime — the wrapper awaits
  it and its completion triggers teardown; early return = teardown
  at birth. Noted on the OpenHandler type docs and both registration
  entry points.
- Breaking at 0.6.0 (the point of landing it before alktunnels
  Phase 1): Ok(Establishment {}) sites become
  Ok(Establishment::default()) mechanically.

Verification: cargo test (621 passed, +4: plan-flows-to-handler,
concurrent same-resource opens get distinct plans, no-establisher
None plan, Establishment construction), clippy -D warnings, fmt
--check, doc clean, test --all-features clean.

src/channels/client.rs

@@ -897,7 +897,7 @@ mod tests {
             Arc::clone(&policy) as Arc<dyn ChannelLifecyclePolicy>;
         let policy_for_assert = Arc::clone(&policy);
 
-        let open_handler: OpenHandler = Arc::new(|_input, _channel_conn, _auth| {
+        let open_handler: OpenHandler = Arc::new(|_input, _plan, _channel_conn, _auth| {
             tokio::spawn(async move {
                 // No-op: the channel's BiStream is available via
                 // `_channel_conn.accept_bi()` if the test wanted
@@ -1220,7 +1220,7 @@ mod tests {
 
         let (data_tx, mut data_rx) = tokio::sync::mpsc::channel::<Vec<u8>>(1);
 
-        let open_handler: OpenHandler = Arc::new(move |_input, channel_conn, _auth| {
+        let open_handler: OpenHandler = Arc::new(move |_input, _plan, channel_conn, _auth| {
             let data_tx = data_tx.clone();
             tokio::spawn(async move {
                 let mut bidi = channel_conn.accept_bi().await.expect("accept_bi");
@@ -1340,7 +1340,7 @@ mod tests {
         let policy_for_assert = Arc::clone(&policy);
 
         let open_handler: OpenHandler =
-            Arc::new(|_input, _channel_conn, _auth| tokio::spawn(async {}));
+            Arc::new(|_input, _plan, _channel_conn, _auth| tokio::spawn(async {}));
         let establisher: OpenEstablisher = Arc::new(|_input, _auth| {
             Box::pin(async {
                 Err(EstablishmentError::DialFailed {
@@ -1472,7 +1472,7 @@ mod tests {
 
         let (data_tx, mut data_rx) = tokio::sync::mpsc::channel::<Vec<u8>>(1);
 
-        let open_handler: OpenHandler = Arc::new(move |_input, channel_conn, _auth| {
+        let open_handler: OpenHandler = Arc::new(move |_input, _plan, channel_conn, _auth| {
             let data_tx = data_tx.clone();
             tokio::spawn(async move {
                 let mut bidi = channel_conn.accept_bi().await.expect("accept_bi");
@@ -2333,7 +2333,7 @@ mod tests {
         let policy_for_hook: Arc<dyn ChannelLifecyclePolicy> =
             Arc::clone(&policy) as Arc<dyn ChannelLifecyclePolicy>;
 
-        let open_handler: OpenHandler = Arc::new(|_input, _channel_conn, _auth| {
+        let open_handler: OpenHandler = Arc::new(|_input, _plan, _channel_conn, _auth| {
             tokio::spawn(async move {
                 tokio::time::sleep(std::time::Duration::from_millis(50)).await;
             })