ParticipantTracker now accepts NdiSourceKind.ActiveSpeaker (Teams' auto-mix output — legacy 'MACHINE (Teams)' or current 'MACHINE (MS Teams - Active Speaker)') and surfaces it as a synthetic row in the participant list with the display name 'Active Speaker'. The operator can route it to its own normalized ISO via the same toggle every other participant uses, so vMix / OBS / Ross can subscribe to a single clean active-speaker feed.
Stable Id: derived from SHA1 of 'auto-mix:<machine>' formatted as a v5 GUID, so a discovery cycle that re-adds the source doesn't duplicate the row and the operator's ISO assignment stays bound across the rename window.
Tests: 78/78 unit (was 76) — added ParticipantTrackerTests.ActiveSpeaker_AppearsAsSyntheticAutoMixParticipant + ActiveSpeaker_ReAddOnSameMachine_PreservesId. Existing NonParticipantSources_AreIgnored still passes (only ActiveSpeaker is opted in; ScreenShare and Audio are still ignored).
