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>

PROGRESS.md

@@ -10,6 +10,28 @@ up instantly. Newest status at the top.
 
 ## ▶ Where we left off / next action
 
+- **Done:** **Fixed a *second* silent-playback bug — the playout clock free-ran and drifted off
+  the stream** (2026-06-17, reported live: both `vccli` and the Windows client showed `talking=1/0`
+  correctly on VAD/PTT, mic + screen-share were recognized by peers, but nothing was audible).
+  Root cause: `RemoteStream::playout_ts` was only ever seeded to `0` and then advanced one Opus
+  frame per playback callback **via the PLC path too** (`core/src/audio/audio_engine.cpp`
+  `on_playback`), so it free-ran at ~1× wall-clock regardless of whether the sender was
+  transmitting. The sender's frame timestamps only advance while it actually sends (the VAD/PTT
+  gate in `core/src/core/client.cpp` 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. The M3 E2E test missed it because clients there talked
+  continuously right after joining, keeping the clocks aligned. **Fix:** `JitterBuffer` gained
+  `peek_front_ts()` (try-lock, RT-safe); `on_playback` now seeds/re-syncs `playout_ts` to the
+  earliest buffered frame on the first frame and whenever it has drifted past ±200/500 ms
+  (`kResyncAheadSamples`/`kResyncBehindSamples`), which both seeds startup and recovers after every
+  silence gap. New regression test `test_playout_resync` (`tests/test_vad_ptt_devices.cpp`):
+  free-runs the clock ~2 s past the drop window, pushes a `ts=0` frame, asserts audible output —
+  verified to fail (energy=0) with the fix disabled, pass (energy≈15M) with it. `ctest --test-dir
+  build/m1-dev` — **14/14 green** (run via PowerShell; Git Bash exec gotcha for these binaries, see
+  `docs/building.md`). **Not yet confirmed audible by ear** — pending the user re-running their
+  live test.
 - **Done:** **Fixed silent-playback bug in `AudioEngine::on_playback`** (2026-06-16, found via
   live manual test: two `vccli --voice` clients, control-plane events and VAD all correct, but
   zero audible output). Root cause: `opus_decode()`'s `max_samples` was being passed the