TASK 21 slice 2a: native-FPS probe seam (ffprobe parse + derive sibling)

- FfmpegFrameRateParser (pure): prefers avg_frame_rate= then r_frame_rate=,
  rational N/N/M, unknown/0 -> null.
- IFrameRateProbe + FfmpegFrameRateProbe: derives sibling ffprobe.exe from the
  located ffmpeg dir, reuses the IDecodeProcess seam for the ffprobe subprocess
  text; null if ffprobe absent.
- FfmpegLocator now also extracts ffprobe.exe (ProbeFileName) from the pinned
  archive, conditional so old caches without it degrade to no pacing.
- Tests: 6 pure parser units + 1 probe integration via fake locator/process;
  FfmpegLocatorTests still green. 15/15, 0 warnings.

Pacing (probe->delay) is slice 2b. Derivative reference: standard ffprobe
avg_frame_rate probing used across OBS/media tooling.

TASKS.md

@@ -1042,6 +1042,7 @@ The tasks below are ordered by dependency and risk. Each task builds on the prev
 2. ✅ `SourceType.MediaSource` addition to the enum
 3. ✅ `MediaSourceModel`: `MediaPath`, `MediaIsLooping`, `MediaVolume` (0-1), `MediaPlaybackState` — persisted in LayoutStore (schema migration + SELECT/INSERT + round-trip test)
 4. ✅ `MediaVideoSource` implements `IMediaFrameSource`: FFmpeg raw video decoder → `VideoFrame` pipeline — spawns ffmpeg `-f rawvideo -pix_fmt bgra`, drains the pipe via pure `RawVideoFrameReader` (`Services/RawVideoFrameReader.cs`), raises `FrameAvailable`/`Completed`; lifecycle is `StartAsync`/`StopAsync`; decode process behind `IDecodeProcess`/`FfmpegDecodeProcess` seam (binary-stdout mirror of `IEncoderProcess`). Shipped 2026-08-31; refactored onto `IMediaFrameSource` with this slice.
+4b. ✅ Slice 2a — native-FPS probe seam: `FfmpegFrameRateParser` (pure, prefers `avg_frame_rate=` then `r_frame_rate=`, rational N/N/M, unknown→null) + `IFrameRateProbe`/`FfmpegFrameRateProbe` (derives sibling `ffprobe.exe` from the located ffmpeg dir, reuses the `IDecodeProcess` seam; null if ffprobe absent) + `FfmpegLocator.ProbeFileName` now also extracts `ffprobe.exe` from the pinned archive (conditional). Tests: `FfmpegFrameRateParserTests` (6 pure units + 1 probe integration via fake locator/process) + `FfmpegLocatorTests` still green — 15/15.
 5. ✅ `IMediaFrameSource` + `MediaVideoSourceManager` (`Services/IMediaFrameSource.cs`, `Services/MediaVideoSourceManager.cs`): app-wide decode-session ownership refcounted by `MediaPath` with a `Func<string, IMediaFrameSource?>` factory seam; `AcquireAsync`/`ReleaseAsync`/`ReleaseAllAsync`/`GetLatestFrame`; coalesces frames onto the UI dispatcher onto a single shared `WriteableBitmap` per file (mirror of `ScreenCaptureManager`); `MediaFailed` + `PreviewBitmapChanged`. Unit tests + one integration test (bitmap share/coalesce) — `MediaVideoSourceManagerTests`, 5/5 pass.
 6. 🔶 Native-FPS pacing — frames currently emit as fast as the pipe produces them; ffprobe FPS probe + per-frame pacing seam still open (slice 2).
 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)**