feat: producer-listen — listen establisher (shape 2) + AcceptQueue contract

- AcceptFn: the injected accepted-connection source (the assembly
  layer owns the listener + accept loop; the protocol never binds —
  OQ-TN-04).
- AcceptQueue: the protocol-side queue contract (async push/pop/
  close; FIFO always-before-take ordering; close-while-waiting
  resolves None). Empty-queue posture is assembly-owned (a late
  accept is legitimate; resource_shortage is the closure's mapping;
  the wrapper's 10s establishment timeout is the backstop).
- listen_establisher: same open op, same params, same typed errors —
  registry namespace gate → accept() → Establishment::new(plan);
  the pump handler is untouched (plan-flow with a different source).
- register_tunnel_listen_openable: same spec/pump registration with
  the listen establisher (the honest shape vs a dial/accept enum:
  the establisher is the only difference; one establisher per op id
  per session registry — documented).
- Tests (tests/producer_listen.rs, 6): listen flow end-to-end, FIFO
  ordering across two opens (R-01 plan-flow, listen-flavored), empty
  queue -> resource_shortage, closed listener -> dial_failed,
  unknown resource -> unknown_resource, late-push wait-then-resolve.
- Harness: RegistrationMode enum + wire_listen.
- TargetHandle gains a structural Debug impl (test ergonomics).

Verified: cargo test green (44), clippy -D warnings (native + wasm32),
fmt clean, wasm32 check passes.

tests/producer_open_op.rs

@@ -188,7 +188,7 @@ async fn open_denied_without_any_identity() {
         .await;
     let topo = wire_with(
         registry,
-        echo_dial(),
+        harness::RegistrationMode::Dial(echo_dial()),
         None,
         None,
         Arc::new(alkcall::core::auth::NoopIdentityProvider),
@@ -239,7 +239,7 @@ async fn open_token_overrides_transport_identity() {
         .await;
     let topo = wire_with(
         registry,
-        echo_dial(),
+        harness::RegistrationMode::Dial(echo_dial()),
         None,
         None,
         Arc::new(harness::TestIdProvider),