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/engine/probe_test.go

@@ -0,0 +1,96 @@
+package engine
+
+import "testing"
+
+func TestDecideAction(t *testing.T) {
+	cases := []struct {
+		name string
+		p    StreamProbe
+		want TranscodeAction
+	}{
+		{
+			name: "MP4 + h264 + AAC = passthrough",
+			p:    StreamProbe{VideoCodec: "h264", AudioCodec: "aac", Container: ".mp4"},
+			want: ActionPassthrough,
+		},
+		{
+			name: "MKV + h264 + AAC = remux",
+			p:    StreamProbe{VideoCodec: "h264", AudioCodec: "aac", Container: ".mkv"},
+			want: ActionRemux,
+		},
+		{
+			name: "MKV + h264 + AC3 = remux audio",
+			p:    StreamProbe{VideoCodec: "h264", AudioCodec: "ac3", Container: ".mkv"},
+			want: ActionRemuxAudio,
+		},
+		{
+			name: "MP4 + h264 + EAC3 = remux audio",
+			p:    StreamProbe{VideoCodec: "h264", AudioCodec: "eac3", Container: ".mp4"},
+			want: ActionRemuxAudio,
+		},
+		{
+			name: "MKV + HEVC = transcode video",
+			p:    StreamProbe{VideoCodec: "hevc", AudioCodec: "aac", Container: ".mkv"},
+			want: ActionTranscodeVideo,
+		},
+		{
+			name: "MP4 + AV1 = transcode video",
+			p:    StreamProbe{VideoCodec: "av1", AudioCodec: "aac", Container: ".mp4"},
+			want: ActionTranscodeVideo,
+		},
+		{
+			name: "h264 10-bit = transcode video (browser refuses)",
+			p:    StreamProbe{VideoCodec: "h264", AudioCodec: "aac", BitDepth: 10, Container: ".mp4"},
+			want: ActionTranscodeVideo,
+		},
+		{
+			name: "h264 + HDR10 = transcode video",
+			p:    StreamProbe{VideoCodec: "h264", AudioCodec: "aac", HDR: "HDR10", Container: ".mp4"},
+			want: ActionTranscodeVideo,
+		},
+		{
+			name: "AVI + h264 + AAC = remux",
+			p:    StreamProbe{VideoCodec: "h264", AudioCodec: "aac", Container: ".avi"},
+			want: ActionRemux,
+		},
+		{
+			name: "Unknown codec = transcode video",
+			p:    StreamProbe{VideoCodec: "mpeg4", AudioCodec: "mp3", Container: ".avi"},
+			want: ActionTranscodeVideo,
+		},
+		{
+			name: "Empty probe falls through to transcode (unknown codec)",
+			p:    StreamProbe{},
+			want: ActionTranscodeVideo,
+		},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			got := DecideAction(&tc.p)
+			if got != tc.want {
+				t.Errorf("got %s, want %s", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestDecideActionNil(t *testing.T) {
+	if DecideAction(nil) != ActionPassthrough {
+		t.Error("nil probe should default passthrough")
+	}
+}
+
+func TestLowerExt(t *testing.T) {
+	cases := map[string]string{
+		"foo.MP4":              ".mp4",
+		"path/to/movie.MKV":    ".mkv",
+		"weird.name.with.dots": ".dots",
+		"":                     "",
+		"noext":                "",
+	}
+	for in, want := range cases {
+		if got := lowerExt(in); got != want {
+			t.Errorf("lowerExt(%q) = %q want %q", in, got, want)
+		}
+	}
+}