fix(review-008 audit): adopted-entry drop guard; explicit-ALPN guards; review 009

Post-landing audit of the 0.7.1 -> 0.8.0 remediation diff: two
hardening guards, one rejection-posture fix, two log/message
corrections, and the deferred coverage debt filed as review 009.

- RelayPlan owns the producer-leg ChannelManager and reclaims the
  adopted spoke channel_id via a Drop guard (replaces the pump
  handler's post-pump_bidi explicit reclaim). Closes the leak
  windows the pump's normal path cannot reach: the wrapper's
  establishment bound expiring after the adopt, and the pump
  handler's early-return arms (plan absent, downcast failure,
  try_unwrap failure, accept_bi failure). The send-half drop still
  EOFs the spoke leg via the mux pump's implicit-EOF sentinel, so
  the spoke-side cascade is unchanged. ADR-051 §6 documents the
  closed post-adopt window (the pre-adopt §6 window and the
  inside-adopt_channel cancellation point stay as documented).
- rebuild_spec_for trims and rejects empty/whitespace
  channel_open_alpn strings — an empty explicit string previously
  overrode a sane name-derived ALPN.
- op_name_is_standard_channel_open_shape applies the same
  empty-segment guard as the derivation: channels//sub no longer
  serializes boolean-only and then reconstructs unmarked (silent
  stub for a marked op); the explicit string rides instead.
- reserved_reply_key_call_error interpolates RESERVED_REPLY_KEY;
  the establisher-bug log fires at warn! (programming error).
- Regression tests: the plan drop guard, the empty-ALPN fallback,
  the empty-segment shape check (672 tests, 3 new).
- CHANGELOG [Unreleased] entry for the audit fixes.
- docs/reviews/009 — the audit's deferred test-coverage gaps
  (template failure arms, filtered/only, batch reserved key,
  wire failure path, golden pins, derivation edge shapes, builder
  overwrite semantics), each with the test to add and gates.

Verification: cargo test 672 passed; clippy --all-targets -D
warnings clean; fmt --check clean; doc --no-deps clean; wasm32
check clean.

docs/architecture/decisions/051-channel-relay-and-hub-leg.md

@@ -214,6 +214,23 @@ relays. The end consumer never authenticates to the spoke directly.
   the window deterministically by severing the spoke leg and asserts
   the spoke-side reclaim + ledger decrement. A channel never outlives
   its leg lifetimes; never a leak past the connection's lifetime.
+- **Adopted-entry teardown on every plan path** — the hub's
+  producer-leg entry (the `adopt_channel` state) is owned by the
+  relay's `RelayPlan` via a `Drop` guard, so it reclaims whenever the
+  plan dies: the normal pump completion, the pump handler's
+  early-return arms (plan absent, downcast failure, `try_unwrap`
+  failure, `accept_bi` failure), and the wrapper's establishment
+  bound expiring after the adopt (the plan dropped before the pump
+  ever spawns). Dropping the plan's send half also EOFs the spoke
+  side through the mux pump's implicit-EOF sentinel (REQ-CH-01), so
+  the spoke handler reclaims through the same cascade the normal
+  path   uses. This closes the adopt-then-timeout window the landing
+  review surfaced (the §6 window above is the *pre-adopt* window;
+  this is the *post-adopt* one, now closed by construction). The
+  one remaining cancellation point *inside* `adopt_channel`'s own
+  await is the same pre-existing client-side window
+  (`ChannelClient::open_channel`'s adopt has it too) — bounded by
+  the leg's transport-EOF `clear_all`, not by the plan.
 - **Multi-hop relays** — a chain of relays composes (each hop is a
   hub leg pair), with the compounding-bound note above. No special
   machinery.