fix(audio): seed/re-sync playout clock so VAD/PTT gaps don't silence playback

RemoteStream::playout_ts was seeded to 0 and only advanced inside the
decode loop (including on every PLC iteration), so it free-ran at ~1x
wall-clock regardless of whether the sender was transmitting. The
sender's frame timestamps only advance while it actually sends (the
VAD/PTT gate returns before ls.timestamp += samples). Across a late join
or any VAD/PTT silence gap the two clocks diverged without bound; once
past the jitter buffer's 500 ms late-drop window every real frame was
dropped-as-late (clock ahead) or never-due (clock behind) -> permanent
silence, while the talk indicator (driven by push_recv_frame, independent
of the jitter buffer) stayed lit.

Add JitterBuffer::peek_front_ts() (try-lock, RT-safe) and seed/re-sync
playout_ts to the earliest buffered frame on the first frame and whenever
it has drifted past +/-200/500 ms. This seeds startup and recovers after
every silence gap.

New regression test test_playout_resync free-runs the clock ~2 s past the
drop window, pushes a ts=0 frame, and asserts audible output: fails
(energy=0) without the fix, passes with it. ctest --preset m1-dev: 14/14.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

core/src/audio/audio_engine.h

@@ -55,6 +55,11 @@ class JitterBuffer {
     // Drops frames that are too old (more than kLateDropSamples late).
     std::optional<Frame> pop(uint32_t playout_ts);
 
+    // Timestamp of the earliest buffered frame, or nullopt if empty/contended. Lets the playout
+    // clock seed/re-sync itself to the arriving stream rather than free-running (see
+    // AudioEngine::on_playback). Uses try_lock — never blocks the real-time callback.
+    std::optional<uint32_t> peek_front_ts() const;
+
     uint32_t target_depth_ms()const { return target_depth_ms_.load(); }
     uint32_t packets_lost()   const { return lost_.load(); }
     void     reset();
@@ -259,6 +264,11 @@ class AudioEngine {
         float            gain  = 1.0f;
         bool             mute  = false;
         uint32_t         playout_ts = 0;
+        // playout_ts free-runs (advances every callback via PLC), so it must be seeded from, and
+        // periodically re-synced to, the actual stream timeline — otherwise it drifts past the
+        // jitter buffer's drop window across VAD/PTT gaps and late joins and every frame is
+        // dropped/never-due (silent playback). false until the first frame seeds it (on_playback).
+        bool             playout_started = false;
 
         // M3: listener-chosen, local-only noise reduction (docs/voice.md §10). Lazily
         // created only when enabled — bounded by how many remote streams this listener