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_test.go

@@ -120,3 +120,54 @@ func TestJPEGEncodeQualityClamp(t *testing.T) {
 		}
 	}
 }
+
+// TestJPEGEncodeBGRA checks the raw-BGRA fast path produces a decodable
+// JPEG with correct SOI/EOI markers.
+func TestJPEGEncodeBGRA(t *testing.T) {
+	enc, err := NewJPEGEncoder()
+	if err != nil {
+		t.Fatalf("NewJPEGEncoder: %v", err)
+	}
+	defer enc.Close()
+
+	// 16x8 BGRA: top half red (R,G,B=255,0,0), bottom half blue (0,0,255).
+	w, h := 16, 8
+	pix := make([]byte, w*h*4)
+	for y := 0; y < h; y++ {
+		for x := 0; x < w; x++ {
+			off := (y*w + x) * 4
+			if y < h/2 {
+				pix[off], pix[off+1], pix[off+2], pix[off+3] = 0, 0, 255, 255 // B,G,R,A = blue,red
+			} else {
+				pix[off], pix[off+1], pix[off+2], pix[off+3] = 255, 0, 0, 255 // B,G,R,A = red,blue
+			}
+		}
+	}
+
+	buf, err := enc.EncodeBGRA(pix, w, h, 85)
+	if err != nil {
+		t.Fatalf("EncodeBGRA: %v", err)
+	}
+	if len(buf) == 0 {
+		t.Fatal("empty JPEG output")
+	}
+	if !bytes.Equal(buf[:2], []byte{0xFF, 0xD8}) {
+		t.Errorf("bad JPEG SOI marker: %x", buf[:2])
+	}
+	if !bytes.Equal(buf[len(buf)-2:], []byte{0xFF, 0xD9}) {
+		t.Errorf("bad JPEG EOI marker")
+	}
+}
+
+// TestJPEGEncodeBGRAShort rejects truncated pixel buffers.
+func TestJPEGEncodeBGRAShort(t *testing.T) {
+	enc, err := NewJPEGEncoder()
+	if err != nil {
+		t.Fatalf("NewJPEGEncoder: %v", err)
+	}
+	defer enc.Close()
+
+	if _, err := enc.EncodeBGRA(make([]byte, 10), 16, 8, 80); err == nil {
+		t.Error("short BGRA buffer accepted")
+	}
+}