Native capture helpers

Windows native recording is resolved from one of these locations:

OPENSCREEN_WGC_CAPTURE_EXE, for local development and diagnostics.
electron/native/wgc-capture/build/wgc-capture.exe, for a locally built Ninja helper.
electron/native/wgc-capture/build/Release/wgc-capture.exe, for a locally built multi-config helper.
electron/native/bin/win32-x64/wgc-capture.exe or electron/native/bin/win32-arm64/wgc-capture.exe, for packaged prebuilt helpers.

Build the Windows helper with:

npm run build:native:win

The build writes the CMake output to electron/native/wgc-capture/build/wgc-capture.exe and copies the redistributable binary to electron/native/bin/win32-x64/wgc-capture.exe.

The helper contract is process-based: the app starts the process with one JSON argument and sends commands on stdin. stop\n finalizes the recording. During migration the helper prints both newline-delimited JSON events and the legacy text messages Recording started / Recording stopped. Output path: <path>.

Current V2 JSON shape:

{
  "schemaVersion": 2,
  "recordingId": 123,
  "sourceType": "display",
  "sourceId": "screen:0:0",
  "displayId": 1,
  "outputPath": "C:\\path\\recording-123.mp4",
  "videoWidth": 1920,
  "videoHeight": 1080,
  "fps": 60,
  "captureSystemAudio": false,
  "captureMic": false,
  "microphoneDeviceId": "default",
  "microphoneGain": 1.4,
  "webcamEnabled": false,
  "outputs": {
    "screenPath": "C:\\path\\recording-123.mp4",
    "webcamPath": "C:\\path\\recording-123-webcam.mp4"
  }
}

The current helper implementation supports display video capture, system audio loopback, and initial default-microphone capture. Webcam and window capture now fail explicitly in the helper rather than silently falling back to Electron capture on Windows. See docs/engineering/windows-native-recorder-roadmap.md for the phased implementation plan.

Smoke-test the helper with:

npm run test:wgc-helper:win
npm run test:wgc-audio:win
npm run test:wgc-mic:win
npm run test:wgc-mixed-audio:win