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).

apps/chanora_flutter/test/beta_e2e_test.dart

@@ -37,6 +37,11 @@ void main() {
 
     await rust.voiceJoin(channelId: snap.channels.first.id, password: '');
 
+    // Zero out the release tail so set_ptt(false) takes effect
+    // synchronously — the default 200 ms tail (SDD-096) would
+    // otherwise delay the assertion below.
+    await rust.setReleaseTailMs(ms: 0);
+
     // Initial stats: PTT off, no frames sent yet.
     final s0 = await rust.audioStats();
     expect(s0.pttActive, isFalse);
@@ -55,6 +60,8 @@ void main() {
     expect(s1.pttActive, isTrue);
 
     await rust.setPtt(active: false);
+    // Give the release-tail (set to 0 above) one tick to settle.
+    await Future<void>.delayed(const Duration(milliseconds: 50));
     final s2 = await rust.audioStats();
     expect(s2.pttActive, isFalse);