feat: add PipeWire screen and system audio capture (M2)

Capture the Wayland desktop via xdg-desktop-portal + PipeWire using
go2tv.app/screencast (MIT), and stream it to OBS:
- internal/capture: Capture/FrameSource/AudioSource interfaces and the
  PipeWire backend (BGRA frames at monitor resolution, S16 48 kHz stereo
  system audio)
- protocol: EncodeBGRA fast path producing 4:2:0 YCbCr JPEGs
- cmd: --source screen|pattern, --audio, --stream-index flags; real
  capture feeds the existing sender
- share one wall-clock reference between the audio and video loops so
  OBS receives aligned A/V timestamps (avoids multi-second latency)

Verified end-to-end: real desktop at 30 fps renders in OBS with
sub-second latency.

internal/protocol/jpeg.go

@@ -74,6 +74,47 @@ func (e *JPEGEncoder) Encode(img image.Image, quality int) ([]byte, error) {
 	}
 }
 
+// EncodeBGRA compresses a raw BGRA (blue, green, red, alpha) pixel buffer of
+// the given dimensions. This is the fast path for the PipeWire screen-capture
+// backend, which delivers frames in BGRA byte order. Subsampling defaults to
+// 4:2:0 (a YCbCr JPEG), so the OBS receiver decodes it as I420 — the same
+// layout obs-teleport produces for OBS-native YCbCr frames.
+func (e *JPEGEncoder) EncodeBGRA(pix []byte, width, height, quality int) ([]byte, error) {
+	if quality < 1 {
+		quality = 1
+	}
+	if quality > 100 {
+		quality = 100
+	}
+
+	want := width * height * 4
+	if len(pix) < want {
+		return nil, errors.New("turbojpeg: BGRA buffer too small")
+	}
+
+	C.tj3Set(e.ctx, C.TJPARAM_QUALITY, C.int(quality))
+	C.tj3Set(e.ctx, C.TJPARAM_SUBSAMP, C.TJSAMP_420)
+	C.tj3Set(e.ctx, C.TJPARAM_COLORSPACE, C.TJCS_YCbCr)
+
+	size := C.tj3JPEGBufSize(C.int(width), C.int(height), C.TJSAMP_420)
+	buf := make([]byte, int(size))
+
+	srcPtr := unsafe.Pointer(&pix[0])
+	dstPtr := (*C.uchar)(&buf[0])
+
+	var pin runtime.Pinner
+	pin.Pin(srcPtr)
+	pin.Pin(dstPtr)
+	defer pin.Unpin()
+
+	jpegSize := size
+	rc := C.tj3Compress8(e.ctx, (*C.uchar)(srcPtr), C.int(width), 0, C.int(height), C.TJPF_BGRA, &dstPtr, &jpegSize)
+	if rc != 0 {
+		return nil, errors.New("turbojpeg BGRA compress failed")
+	}
+	return buf[:int(jpegSize)], nil
+}
+
 // encodeRGBA compresses a Go RGBA image (pixel layout [R,G,B,A] per 4 bytes).
 // TJPF_RGBA tells turbojpeg the exact layout; colourspace is RGB.
 func (e *JPEGEncoder) encodeRGBA(img *image.RGBA) ([]byte, error) {