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>

scripts/livekit-dev-room.py

@@ -6,7 +6,7 @@ Usage:
     eval "$(python3 scripts/livekit-dev-room.py)"
     ctest --test-dir build -R test_integration_livekit --output-on-failure
 
-Prints shell `export` lines for the four STPLUGIN_IT_* variables
+Prints shell `export` lines for the STPLUGIN_IT_* variables
 core/tests/test_integration_livekit.cpp looks for. With no arguments it uses
 `livekit-server --dev`'s built-in devkey/secret credentials.
 
@@ -58,6 +58,7 @@ def main() -> None:
     parser.add_argument("--room", default="obs-plugin-it")
     parser.add_argument("--publisher-identity", default="cam-test")
     parser.add_argument("--subscriber-identity", default="obs:obs-plugin-it:test")
+    parser.add_argument("--bystander-identity", default="cam-bystander")
     args = parser.parse_args()
 
     publish_token = mint(args.api_key, args.api_secret, args.publisher_identity, args.room,
@@ -68,10 +69,16 @@ def main() -> None:
     subscribe_token = mint(args.api_key, args.api_secret, args.subscriber_identity, args.room,
                            publish=False, subscribe=True)
 
+    # A second camera in the same room that the session must NOT subscribe
+    # to -- the selective-subscription check.
+    bystander_token = mint(args.api_key, args.api_secret, args.bystander_identity, args.room,
+                           publish=True, subscribe=False)
+
     print(f'export STPLUGIN_IT_URL="{args.url}"')
     print(f'export STPLUGIN_IT_PUBLISH_TOKEN="{publish_token}"')
     print(f'export STPLUGIN_IT_SUBSCRIBE_TOKEN="{subscribe_token}"')
     print(f'export STPLUGIN_IT_PUBLISHER_IDENTITY="{args.publisher_identity}"')
+    print(f'export STPLUGIN_IT_BYSTANDER_TOKEN="{bystander_token}"')
 
 
 if __name__ == "__main__":