fix: session replay memory leak

[marandaneto]

💡 Motivation and Context

Fixes #142

Session replay on Flutter has multiple memory leak vectors:

1. Overlapping snapshot captures: _generateSnapshot() was fire-and-forget (async but not awaited), so if a capture took longer than the 1s throttle, multiple captures would run concurrently — each allocating large ui.Image objects, raw RGBA byte arrays (~8MB each), PictureRecorders, etc.
2. Full raw RGBA bytes stored permanently for diffing: Each captured frame stored a full raw RGBA Uint8List (~8MB for a 1080×1920 screen) in ViewTreeSnapshotStatus.imageBytes just to compare with the next frame.
3. Native resources leaked on errors: The catchError handler didn't dispose ui.Image, Picture, or PictureRecorder objects when exceptions occurred mid-processing.
4. Per-frame callbacks: ChangeDetector registered an addPostFrameCallback on every single frame (~60/s), even though all but ~1/s were immediately rejected by the throttle.
5. No cancellation of in-flight work: Stopping recording didn't cancel ongoing captures, which continued allocating and processing images.

💚 How did you test it?

• All 93 existing unit tests pass
• flutter analyze reports no issues
• Manual verification of code paths for resource disposal

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file
• Added the release label to the PR

[marandaneto]

didnt test this yet but this is what claude thinks about the linked issue