[Frameforge 5] Implement canvas capture, MediaStream tracks and real MediaRecorder export

[danwalmsley]

Outcome and dependencies

Depends on #54, #55, #57, #34 and #40. Export the original app's composited movie plus mixed audio, including cancellation. MediaRecorder and the media stream path need implementation/qualification; this is not offline deterministic mastering.

Specification and implementation

• Implement canvas.captureStream(frameRate), CanvasCaptureMediaStreamTrack as applicable, MediaStream getTracks/getAudioTracks/addTrack and MediaStreamTrack clone/stop/enabled/readyState. Connect the Web Audio destination track from [Frameforge 2] Implement Web Audio mixing, media sources and waveform decoding #55; one clone stopping must not terminate its siblings.
• Implement MediaRecorder constructor/options, isTypeSupported, mimeType/state, start(timeslice), stop, pause/resume/requestData and required dataavailable/stop/error events and BlobEvent bytes. Follow state/error and final-chunk ordering; capability probing reflects an available codec/container encoder and muxer, not a hardcoded list.
• Qualify at least one actual Frameforge-probed format on macOS (e.g. H.264/AAC MP4 if supported by the chosen backend). Explicitly list supported codec/profile/container combinations and fail others. Reuse a cross-platform encoder interface with backend-specific acceleration; document licenses, pinned dependencies and runtime package contents.
• Preserve frame generations across GPU -> Canvas2D drawImage -> capture -> encoder. Prefer shared GPU surfaces or GPU conversion into encoder-compatible surfaces; do not introduce per-frame CPU readback into preview. Count unavoidable copy/conversion costs separately. Explicit PNG readback is separate from the video capture path.
• Use timestamped bounded video/audio queues, backpressure, a shared recording timebase and cancellation. Encoding/muxing must not block JS/UI or realtime audio. Correctly settle recorder events when tracks end, device/encoder errors occur, document hides/closes or source size changes.
• Exercise Frameforge's timeslice=1000, final tail, hidden-tab cancellation, repeated exports, resolution/playhead restoration and cleanup. App chunks remain memory-resident by upstream design; measure and respect that limitation rather than claim unlimited export duration.

Tests and verification

Record known moving color bars plus click track. Independently demux/decode output and inspect dimensions, codecs, monotonic timestamps, duration, non-repeated content and A/V alignment. Test unsupported MIME, no tracks, state misuse, stop-before-first-frame, mid-export cancellation, device loss and closing window. Run two consecutive original app exports and verify playback plus restored preview state. Add MediaRecorder/capture WPT contracts and native queue/muxer tests.

Acceptance

• Original export produces a playable composited movie with mixed audio in a honestly advertised format.
• Cancel/stop/error deliver correct events and release tracks, encoders and frame leases.
• Preview remains GPU resident; measured export cost and A/V/drop results are attached.
• Native AOT bundle contains all non-system dependencies and passes recording regressions.

References

MediaStream Recording and capture from DOM elements.

Original application references

• src/export.js
• src/renderer.js
• src/media.js

Shared delivery requirements

Parent: #53. Baseline: PR #43 / ce56d49; sample experiments/WebScene.Frameforge.
Reference application: https://github.com/wieslawsoltes/Frameforge (pin f414cd44896b659a5c6da2e4fc596a2ded055f2a, includes demo media and THIRD_PARTY_NOTICES.md).
Implement in WebScene, leaving original application JavaScript/WGSL unchanged. No fake capability objects, synthetic successful events or swallowed errors. Keep public contracts cross-platform with isolated backend implementations; qualify macOS arm64 first and explicitly track Windows/Linux follow-up. Production Avalonia 11 must remain supported; only samples opt into Avalonia 12. Native AOT/trim safety is mandatory.
Add focused native/managed regression coverage and a pinned WPT subset or clearly labeled project-owned contract for non-WebGPU behavior. Applicable WebGPU CTS cases belong with GPU work. Record exact revisions, test commands, negative cases and reproducible evidence in the implementation PR. Unsupported functionality cannot be marked passed; distinguish app coverage from standards conformance. Review existing implementation before adding overlapping infrastructure.

Native Save integration

The exported movie Blob must flow through #57 into the hosting Avalonia/Uno native Save workflow, with suggested filename and format, user-selected destination and platform overwrite/cancellation handling. There is no browser download manager or automatic Downloads destination. Verify the resulting on-disk/provider file independently; a completed encoder or created Blob alone is not a successful native save. Keep original application code unchanged and implement the compatibility bridge in WebScene.