feat: stop-motion support (images → video)

[hm21]

Summary

Adds stop-motion support: render a video from a sequence of still images, each frame held for a configurable duration — the classic choppy stop-motion look. This was previously impossible since the whole render pipeline only accepted video sources.

A dedicated feature parallel to render (rather than touching the composition pipeline), so it reuses the existing task/progress/cancel infrastructure and is low-risk.

API

final data = StopMotionRenderData(
  frames: [
    StopMotionFrame(image: EditorLayerImage.file(File('frame1.png'))),
    StopMotionFrame(image: EditorLayerImage.memory(bytes),
        duration: const Duration(milliseconds: 500)), // per-frame override
  ],
  frameRate: 12,               // default frame duration = 1/12s
  fit: StopMotionFit.contain,  // contain | cover | stretch
  // resolution: Size(1080, 1920), // optional; defaults to the first frame size
);

final bytes = await ProVideoEditor.instance.renderStopMotion(data);
// or, memory-friendly for long sequences:
await ProVideoEditor.instance.renderStopMotionToFile(outputPath, data);

The output is silent — add music by passing the result through renderVideo with audioTracks.

Implementation

• Dart: StopMotionRenderData / StopMotionFrame / StopMotionFit; renderStopMotion / renderStopMotionToFile on the platform interface, method channel and web (unsupported) stub.
• Darwin (iOS/macOS): AVAssetWriter + pixel-buffer adaptor; EXIF-aware ImageIO decoding (CGImageSourceCreateThumbnailWithTransform) with downscaling.
• Android: Media3 Transformer image input (setImageDurationMs) with a Presentation effect for the fit mode; ExifInterface orientation handling and inSampleSize downscaled decoding.
• Frame preparation owns the first 20% of the progress bar so it no longer sits at 0% while decoding many photos.
• Reuses RenderTask / RenderJobHandle → cancel(id) works unchanged.

Platform support

Android	iOS	macOS	Windows	Linux	Web
✅	✅	✅	❌	❌	🚫

Tests & verification

• flutter analyze (package + example): clean.
• flutter test: all unit tests pass (model + method-channel coverage).
• New integration test (example/integration_test/stop_motion_test.dart): duration, resolution, per-frame duration, all fit modes, to-file, progress, cancel.
• Native compiles verified: flutter build macos and flutter build apk both succeed.
• Example app gains a Stop-Motion page (synthetic sample frames out-of-the-box or pick images).

Version bumped to 1.20.0.