feat(session): subscribe only to the slot's own camera and mic

Every plugin source connected with auto_subscribe on, so each one pulled
every camera in the room and discarded all but one. In the 2026-10-04
old-gods-of-appalachia show (8 sources, 7 cameras) each source carried
4-7 Mbps and all 8 hit congestion in the same instant: they share the
director's single downlink, so the SFU starved the cameras on screen.

The session now connects with auto_subscribe off and calls
setSubscribed(true) on exactly the wanted publications: at connect, on
onTrackPublished (re-sweeping the participant, since SDK 1.10.1 delivers
that event's publication null), and after a reconnect. Any subscription
it did not ask for is reported and handed back. Audio-only sources no
longer subscribe-then-disable video; they never subscribe to it.

The integration test gains a bystander camera that must never be
subscribed (fails with 3 unwanted subscriptions before this change) and
a full publisher leave/rejoin as a new participant session.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

core/tests/test_session.cpp

@@ -92,6 +92,34 @@ void testTrackSelection()
     ST_ASSERT(!isWantedAudioTrack(want, "other", MediaKind::Audio, MediaSource::Microphone));
 }
 
+void testSubscriptionSelection()
+{
+    const std::string want = "cam1";
+
+    // A camera source subscribes to exactly its slot's camera and mic.
+    ST_ASSERT(shouldSubscribe(want, "cam1", MediaKind::Video, MediaSource::Camera, true, true));
+    ST_ASSERT(shouldSubscribe(want, "cam1", MediaKind::Audio, MediaSource::Microphone, true, true));
+
+    // Every other participant's tracks are the bandwidth this exists to save:
+    // with N sources in one OBS, each of them used to pull the whole room.
+    ST_ASSERT(!shouldSubscribe(want, "cam2", MediaKind::Video, MediaSource::Camera, true, true));
+    ST_ASSERT(!shouldSubscribe(want, "cam2", MediaKind::Audio, MediaSource::Microphone, true, true));
+    // ...and so is the slot's own screenshare.
+    ST_ASSERT(!shouldSubscribe(want, "cam1", MediaKind::Video, MediaSource::Screenshare, true, true));
+    ST_ASSERT(!shouldSubscribe(want, "cam1", MediaKind::Audio, MediaSource::ScreenshareAudio, true, true));
+
+    // An audio-only source (the soundboard) never subscribes to video at all,
+    // rather than subscribing and then disabling it.
+    ST_ASSERT(!shouldSubscribe(want, "cam1", MediaKind::Video, MediaSource::Camera, false, true));
+    ST_ASSERT(shouldSubscribe(want, "cam1", MediaKind::Audio, MediaSource::Microphone, false, true));
+    // A video-only source does not pull the mic.
+    ST_ASSERT(!shouldSubscribe(want, "cam1", MediaKind::Audio, MediaSource::Microphone, true, false));
+    ST_ASSERT(shouldSubscribe(want, "cam1", MediaKind::Video, MediaSource::Camera, true, false));
+
+    // No selection subscribes to nothing.
+    ST_ASSERT(!shouldSubscribe("", "cam1", MediaKind::Video, MediaSource::Camera, true, true));
+}
+
 void testStateMachineHappyPath()
 {
     SessionStateMachine m;
@@ -527,6 +555,7 @@ int main()
 {
     testFrameGeometry();
     testTrackSelection();
+    testSubscriptionSelection();
     testStateMachineHappyPath();
     testPublisherSwapIsNotAnError();
     testReconnect();