feat(ptt,audio): wire PTT key edges through ReleaseTailTimer (SDD-096)

Previously the PttController handed its real AudioTransmitGate to
the platform backend and the backend wrote transmit_active directly
on every key edge — bypassing the 200 ms release tail and the
TransmitMode selector entirely. The tail timer was constructed and
exposed on ChanoraSession but never received any input, so SDD-096
and SRS-206 were spec-only.

Wire it: PttController now owns a synthetic 'press-edge gate' which
it hands to the backend in place of the real one. An internal
edge-watcher task subscribes to that press-gate, translating
true/false transitions into ReleaseTailTimer.key_down/key_up calls.
The release-tail timer feeds the selector's ptt_held input; the
selector recomputes transmit_active honouring mode, in_channel,
and hard_mute, and writes the real gate. Single owner of
transmit_active is preserved (SAD-083 invariant).

PttController::new now takes Arc<ReleaseTailTimer> instead of
AudioTransmitGate; ChanoraSession threads its session-scoped timer
through both the start_audio path and the supervisor reconnect
path. The legacy bridge set_ptt call (still used by the in-focus
Listener fallback and the e2e test) is rerouted through the timer
so the same tail and mute semantics apply uniformly.

ReleaseTailTimer gains force_release() — cancels any pending task
AND clears the selector's ptt_held. PttController::stop uses it so
shutdown can't leave transmit_active stuck at true.

Tests
- press_edge_drives_selector_through_release_tail: backend press
  edge → real gate follows, key_up → tail keeps gate true for tail
  window then clears.
- stop_clears_press_and_cancels_tail: stop() drops transmit even
  with a tail in flight.
- e2e test now sets release_tail_ms=0 + waits one tick so the
  pttActive=false assertion isn't racing the default 200 ms tail.

cargo test --workspace --lib: 74 passed / 0 failed / 1 ignored
(+2 new tests vs. the previous 72).
flutter analyze: clean (6 pre-existing Radio.groupValue infos).

crates/chanora_audio/src/release_tail.rs

@@ -100,6 +100,16 @@ impl ReleaseTailTimer {
         self.cancel_pending();
     }
 
+    /// Cancel any pending release task and immediately clear the
+    /// selector's `ptt_held` input. Used on PTT controller
+    /// shutdown to guarantee `transmit_active` does not get stuck
+    /// at `true` if the user's last action was a key-down with
+    /// no matching key-up reaching us before the shutdown.
+    pub fn force_release(&self) {
+        self.cancel_pending();
+        self.selector.set_ptt_held(false);
+    }
+
     fn cancel_pending(&self) {
         if let Ok(mut g) = self.pending.lock() {
             if let Some(h) = g.take() {