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.

go.mod

@@ -2,9 +2,13 @@ module teleportfling
 
 go 1.26.0
 
-require github.com/schollz/peerdiscovery v1.7.6
+require (
+	github.com/schollz/peerdiscovery v1.7.6
+	go2tv.app/screencast v0.0.0-20260807191623-4cba6136251a
+)
 
 require (
+	github.com/godbus/dbus/v5 v5.1.0 // indirect
 	golang.org/x/net v0.59.0 // indirect
 	golang.org/x/sys v0.48.0 // indirect
 )