feat(stream): pion-based WebRTC byte streamer for browser playback

Replaces the broken anacrolix WebTorrent path with a custom WebRTC peer
that the browser drives directly. Architecture matches plan/clever-
weaving-dove.md (Fase 2 + 3 + 6 of the streaming pivot).

- engine/wire: shared 12-byte binary frame format (Hello / RangeReq /
  RangeData / RangeEnd / Cancel / Ping / Pong / SeekHint). Roundtrip +
  oversized-frame rejection tests.
- agent/signal_client: SSE consumer + POST sender for SDP/ICE relay
  through /api/internal/stream/signal/<id>; auto-reconnects.
- engine/webrtc_stream: pion v4 PeerConnection + DataChannel pump.
  Reads file via os.ReadAt, chunks RangeData at 16 KiB, honours app-
  level backpressure with SetBufferedAmountLowThreshold.
- cmd/daemon dispatcher learns mode webrtc_stream + new
  webrtcSessionRegistry tracks per-session cancel funcs for clean
  shutdown.
- engine/probe + hwaccel + transcoder: foundation for Fase 2.5
  (codec detection, NVENC/QSV/VAAPI/VideoToolbox autodetection,
  ffmpeg pipe wrapper to fragmented MP4). Integration into
  webrtc_stream still pending.
- pion/webrtc/v4 promoted from indirect to direct dep.

End-to-end against unarr-dev confirms a 122 MB 1080p H.264 / AAC MP4
plays in Chrome with the new pipeline.

internal/agent/daemon.go

@@ -35,6 +35,7 @@ type Daemon struct {
 	// Callbacks — set by cmd/daemon.go before calling Run.
 	OnTasksClaimed    func(tasks []Task)
 	OnStreamRequested func(req StreamRequest)
+	OnWebRTCSession   func(sess WebRTCSession)
 	OnControlAction   func(action, taskID string, deleteFiles bool)
 	GetActiveCount    func() int // returns number of active downloads (wired from manager)
 
@@ -169,6 +170,11 @@ func (d *Daemon) Run(ctx context.Context) error {
 			d.OnStreamRequested(req)
 		}
 	}
+	d.sync.OnWebRTCSession = func(sess WebRTCSession) {
+		if d.OnWebRTCSession != nil {
+			d.OnWebRTCSession(sess)
+		}
+	}
 	d.sync.OnUpgrade = func(version string) {
 		if version != d.lastNotifiedVersion {
 			d.lastNotifiedVersion = version