fix: emit JPEG frames for default video streams

[SunsetWan]

Summary

• ensure screenshot-backed streams encode PNG captures as JPEG at the default scale=1.0 / quality=80 settings
• preserve byte-for-byte passthrough when the input is already JPEG
• add shared regression coverage plus iOS and Android MJPEG E2E assertions for MIME, Content-Length, and JPEG magic bytes
• document the fix under Unreleased

Root cause

Both the iOS screenshot API and Android screencap -p produce PNG data. The shared frame processor treated the default scale and quality as an unconditional passthrough fast path, while both MJPEG writers labeled every frame as image/jpeg. This produced PNG payloads with a JPEG MIME type.

The fast path now checks the source container with ImageIO and only passes through actual JPEG input. PNG captures are re-encoded as JPEG. The shared raw and ffmpeg screenshot-backed formats now also consistently emit the JPEG frames their command descriptions promise.

User impact

Strict MJPEG clients no longer receive PNG frames mislabeled as JPEG at default settings. Non-default quality and scale behavior is unchanged.

Validation

• swift test --filter VideoFrameProcessingTests: 9 passed
• make build: passed
• Standards/spec review: passed
• make test: all 306 tests passed, then the Swift test process aborted during teardown with freed pointer was not the last allocation; the same post-test abort reproduces on unchanged origin/main
• Device E2E not run locally: no booted iOS simulator and no adb on PATH

[SunsetWan]

Superseded by lycorp-jp#94.