fix(session): recover stalled video subscriptions with a keyframe-forcing watchdog

Camera sources in the OBS plugin were dropping out at random and never
recovering, while the same players stayed healthy in browser talkback.
Measured on the live server: every OBS plugin subscriber racked up
862-1099 nackMisses (retransmit requests for packets the SFU had already
aged out of its send buffer -- unrecoverable loss) and sat at plis == 2
for a multi-hour session, versus 0/adaptive-PLI for a browser subscriber
in the same room. The pinned client-sdk-cpp (1.10.1) exposes no
PLI/keyframe-request API at all, so a decoder that lost a frame that way
had no way to resync for the rest of the show.

Add a stall-recovery watchdog: StallWatchdog (session_types.h/.cpp) is a
pure, fake-clock-testable class that decides when a video subscription
has gone too long (2000ms, kStallRecoveryTimeout) without a decoded
frame reaching OBS. LiveKitSession::Impl polls it from a dedicated
thread and, through the existing command queue (never touching the SDK
off the worker thread), toggles the publication's setEnabled(false)/
setEnabled(true) -- the one lever this SDK exposes that makes the SFU
restart delivery, and a restart always begins with a keyframe.

Repeated attempts against the same stall back off exponentially
(2s/4s/8s/16s/30s-capped, mirroring the shape of the adapter's own
reconnect backoff) so a genuinely gone publisher is retried on a bounded
cadence instead of hammered every 2 seconds. A muted, disabled or
unsubscribed track, an audio-only source, or a disconnected session
never arms the watchdog: new onTrackMuted/onTrackUnmuted handlers
suspend and resume its clock, always re-baselining from "now" rather
than a stale timestamp, so un-muting after a long legitimate camera-off
period cannot read as a multi-minute stall. Each attempt, and eventual
recovery, is logged through a new DiagnosticHandler that the OBS adapter
maps onto obs_log at the same severities the file already uses for other
notable events.

Also investigated (not changed): the setVideoQuality(HIGH) pin added for
an earlier simulcast-resize bug. The SDK's own docs and the FFI binary's
wire-protocol strings show setVideoQuality only bounds spatial/simulcast
layer selection (UpdateTrackSettings.quality), never temporal layers or
frame rate, which the SFU's own congestion control governs independently
-- so this pin is unlikely to be the cause of the packet-loss symptom,
and is probably an inert no-op now that publishing is pinned server-side
to a single spatial layer (L1T3). Left in place since that is not fully
unambiguous from the SDK alone. Full writeup in
.stall-recovery-report.md (untracked, not part of this commit).

Adds 3 new headless StallWatchdog tests (fires-after-threshold, does-
not-fire-when-muted/disabled, backs-off-rather-than-loops) to
test_session.cpp; caught a real bug during development where
onFrameDelivered() reset the backoff duration but not the
next-attempt-allowed timestamp, throttling a just-recovered stream
against its own stale backoff. Built and all 6 CTest suites pass
(124/124 checks in test_session); also verified clean under
ThreadSanitizer with warning counts unchanged from the pre-change
baseline.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

obs-adapter/src/plugin-main.cpp

@@ -375,6 +375,13 @@ void *sourceCreate(obs_data_t *settings, obs_source_t *source)
 
     self->session->setVideoHandler([self](const VideoFrameData &frame) { outputVideoFrame(self, frame); });
     self->session->setAudioHandler([self](const AudioFrameData &frame) { outputAudioFrame(self, frame); });
+    // The stall-recovery watchdog (core/src/session.cpp) is the only thing
+    // that currently uses this: it logs each toggle-the-subscription
+    // recovery attempt, and its eventual success, so a stalled-and-fixed
+    // camera is diagnosable from an OBS log afterward instead of invisible.
+    self->session->setDiagnosticHandler([](DiagnosticLevel level, const std::string &message) {
+        obs_log(level == DiagnosticLevel::Warning ? LOG_WARNING : LOG_INFO, "%s", message.c_str());
+    });
     self->session->setStateHandler([self](SessionState state, const std::string &detail) {
         self->setStatus(detail.empty() ? describeSessionState(state) : detail);
         self->status_is_error.store(state == SessionState::Failed);