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.

go.mod

@@ -13,6 +13,7 @@ require (
 	github.com/google/uuid v1.6.0
 	github.com/huin/goupnp v1.3.0
 	github.com/olekukonko/tablewriter v1.1.4
+	github.com/pion/webrtc/v4 v4.2.11
 	github.com/spf13/cobra v1.10.2
 	github.com/torrentclaw/go-client v0.2.0
 	golang.org/x/term v0.41.0
@@ -105,7 +106,6 @@ require (
 	github.com/pion/stun/v3 v3.1.1 // indirect
 	github.com/pion/transport/v4 v4.0.1 // indirect
 	github.com/pion/turn/v4 v4.1.4 // indirect
-	github.com/pion/webrtc/v4 v4.2.11 // indirect
 	github.com/pkg/errors v0.9.1 // indirect
 	github.com/protolambda/ctxlock v0.1.0 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect