Add Linux virtual camera output via v4l2loopback

[pubino]

Summary

• Adds --virtual-camera <device> (and --virtual-camera-pix-fmt) so page-stream can write to a Linux v4l2loopback device. The stream then appears as a webcam to any host app (Chromium/Firefox, Zoom, Meet, OBS, GStreamer, etc.).
• Works with both browser/x11grab and --video-file inputs; ffmpeg branches to a raw v4l2 output (no libx264, no audio) when virtual-camera mode is on.
• --ingest is now optional when --virtual-camera is supplied. Health logs and reconnect/backoff machinery understand the v4l2 target.
• Linux-only by design — macOS support requires a CoreMediaIO DAL plugin (or third-party tools like OBS) and is intentionally out of scope for this iteration.

Includes:

• scripts/setup-virtual-camera.sh — zsh-friendly helper to load/unload v4l2loopback with sensible defaults (devices=1 video_nr=10 card_label=PageStream), plus --device/--label/--status/--teardown.
• README "Virtual Camera Output (Linux)" section with install commands, examples, and verification tips (ffplay -f v4l2 /dev/video10).
• 9 new tests in tests/virtual-camera.test.ts covering CLI parsing, ffmpeg arg construction for both input modes, and platform/device guards.

Test plan

Real-world local validation before merging to main:

• Install module: sudo apt install v4l2loopback-dkms
• Load device: sudo ./scripts/setup-virtual-camera.sh (creates /dev/video10, label PageStream)
• Build: docker build -t page-stream:vc . (or run directly with locally-built dist/)
• Page → camera: node dist/index.js --virtual-camera /dev/video10 --url demo/index.html --width 1280 --height 720 --fps 30
• Verify output frames: ffplay -f v4l2 /dev/video10
• Verify in browser: chromium --use-fake-ui-for-media-stream → https://webcamtests.com (or any getUserMedia page) and pick "PageStream" from the camera list
• Verify in a real client app: Zoom / Google Meet / OBS — confirm the device shows up under its label and renders the page
• Looping video → camera: node dist/index.js --virtual-camera /dev/video10 --video-file ./videos/loop.mp4 --video-loop
• Negative path: node dist/index.js --virtual-camera /dev/nonexistent should exit with a clear error pointing at setup-virtual-camera.sh
• Teardown: sudo ./scripts/setup-virtual-camera.sh --teardown

Automated tests (28 pass): docker run --rm --entrypoint /bin/bash page-stream:vc -c "node --test --loader ts-node/esm tests/basic.test.ts tests/crop-infobar.test.ts tests/input-flags.test.ts tests/video-file.test.ts tests/virtual-camera.test.ts"

Notes & follow-ups

• The Docker workflow defaults to network ingests; running virtual-camera mode from inside a container requires --device /dev/video10:/dev/video10 and is documented as such. Most users will run this on the host.
• macOS virtual camera support is a natural follow-up; the architecture there is significantly heavier (CoreMediaIO DAL plugin) and out of scope for this PR.