TASK 21 slice 3: loop control in MediaVideoSource (process factory + Loop flag)

- IMediaFrameSource gains bool Looping.
- MediaVideoSource ctor takes Func<IDecodeProcess> processFactory instead of a
  single IDecodeProcess: a System.Diagnostics.Process can't be re-Start()ed, so
  each loop pass creates a fresh decoder. Decode wrapped in do-while(Looping):
  restart on natural EOF instead of raising Completed.
- Production wiring (MainViewModel media factory): passes the process factory
  AND FfmpegFrameRateProbe -- closes the slice-2b gap where production had no
  probe and therefore no pacing.
- Tests: loop test (single frame re-emits across passes, Completed only when
  loop cleared); fakes updated for the new interface member. Media tests 12/12,
  build 0 warnings.

Wiring Source.MediaIsLooping into the flag needs a manager-level per-path loop
provider -> lands with the UI-picker (acquisition) slice.

Derivative reference: looping media by restarting decode on EOF, standard in
playback/overlay tooling (OBS media source repeat).

TASKS.md

@@ -1047,7 +1047,7 @@ The tasks below are ordered by dependency and risk. Each task builds on the prev
 6. ✅ Native-FPS pacing (slice 2b): `MediaVideoSource` takes optional `IFrameRateProbe?` + `Func<TimeSpan,CancellationToken,Task>? delay` seams, probes FPS once in `RunAsync`, and delays by 1/fps after each emitted frame; unknown/absent probe → no pacing. Test: `MediaVideoSource_PacesFramesByProbedFps` (fake probe + recording delay, one delay per frame ≈ 1ms).
 7. ☐ Wire into `FramePump` resolver — `Source { Type: MediaSource }` → latest video frame — **resolver + preview routing + manager wired; session acquisition (start/stop on add/remove) still open (comes with the UI picker)**
 8. ☐ Wire into `SceneCompositor` — render media source as an image element at its position/size
-9. ☐ Loop control — `IsLooping` property, restart on end
+9. ✅ Loop control (mechanism) — `IMediaFrameSource.Looping`; `MediaVideoSource` takes a `Func<IDecodeProcess>` process factory and restarts the decode on natural EOF when `Looping` (fresh process per pass, since a `Process` can't be re-`Start()`ed). Test: `MediaVideoSource_LoopsUntilLoopDisabled` (single frame re-emits across passes, `Completed` only after loop cleared). Wiring `Source.MediaIsLooping` into the flag lands with the UI-picker (acquisition) slice.
 10. ☐ Volume control — per-source volume slider for audio playback
 11. ☐ UI: file picker (filtered to video formats), loop toggle, volume slider
 12. ☐ Schema migration for media source settings (file path, loop, volume)