TASK 21 Increment A: media-source model + persistence (schema migration + round-trip test)
SourceType.MediaSource added; MediaSourceType (Video/Audio) and MediaPlaybackState
(Stopped/Playing/Paused) enums. Source gains MediaPath, MediaIsLooping, MediaVolume
(0-1 clamped), MediaPlaybackState. LayoutStore: MigrateSourceTable adds 4 guarded
ALTERs (MediaPath, MediaIsLooping, MediaVolume, MediaPlaybackState); SELECT appends
indices 26-29, INSERT adds 4 columns + params. Round-trip test
MediaSource_Config_Survives_Save_And_Reload passes. Build 0 warnings, 245/247
tests (2 known pre-existing). Gated on creator run for later integration (ffmpeg).