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/transcoder_test.go

@@ -0,0 +1,151 @@
+package engine
+
+import (
+	"strings"
+	"testing"
+)
+
+func sliceContains(args []string, want string) bool {
+	for _, a := range args {
+		if a == want {
+			return true
+		}
+	}
+	return false
+}
+
+func sliceContainsPair(args []string, key, val string) bool {
+	for i := 0; i < len(args)-1; i++ {
+		if args[i] == key && args[i+1] == val {
+			return true
+		}
+	}
+	return false
+}
+
+func TestBuildFFmpegArgsPassthroughCopy(t *testing.T) {
+	args := buildFFmpegArgs("/tmp/movie.mp4", TranscodeOpts{
+		Action:     ActionPassthrough,
+		HWAccel:    HWAccelNone,
+		FFmpegPath: "ffmpeg",
+	})
+	if !sliceContainsPair(args, "-c:v", "copy") {
+		t.Errorf("passthrough should keep -c:v copy. args=%v", args)
+	}
+	if !sliceContainsPair(args, "-c:a", "copy") {
+		t.Error("passthrough should keep -c:a copy")
+	}
+	if !sliceContainsPair(args, "-f", "mp4") {
+		t.Error("output container must be mp4")
+	}
+	movflags := ""
+	for i := 0; i < len(args)-1; i++ {
+		if args[i] == "-movflags" {
+			movflags = args[i+1]
+		}
+	}
+	if !strings.Contains(movflags, "frag_keyframe") {
+		t.Errorf("movflags must include frag_keyframe, got %q", movflags)
+	}
+}
+
+func TestBuildFFmpegArgsRemuxAudio(t *testing.T) {
+	args := buildFFmpegArgs("/tmp/movie.mkv", TranscodeOpts{
+		Action:       ActionRemuxAudio,
+		AudioBitrate: "256k",
+		FFmpegPath:   "ffmpeg",
+	})
+	if !sliceContainsPair(args, "-c:v", "copy") {
+		t.Error("remux-audio keeps video copy")
+	}
+	if !sliceContainsPair(args, "-c:a", "aac") {
+		t.Error("remux-audio must transcode audio to aac")
+	}
+	if !sliceContainsPair(args, "-b:a", "256k") {
+		t.Error("audio bitrate override not honored")
+	}
+}
+
+func TestBuildFFmpegArgsTranscodeVideoSoftware(t *testing.T) {
+	args := buildFFmpegArgs("/tmp/movie.mkv", TranscodeOpts{
+		Action:       ActionTranscodeVideo,
+		HWAccel:      HWAccelNone,
+		Preset:       "fast",
+		VideoBitrate: "6M",
+		FFmpegPath:   "ffmpeg",
+	})
+	if !sliceContainsPair(args, "-c:v", "libx264") {
+		t.Error("software fallback must use libx264")
+	}
+	if !sliceContainsPair(args, "-preset", "fast") {
+		t.Error("custom preset not honored")
+	}
+	if !sliceContainsPair(args, "-b:v", "6M") {
+		t.Error("video bitrate not honored")
+	}
+}
+
+func TestBuildFFmpegArgsTranscodeVideoNVENC(t *testing.T) {
+	args := buildFFmpegArgs("/tmp/movie.mkv", TranscodeOpts{
+		Action:     ActionTranscodeVideo,
+		HWAccel:    HWAccelNVENC,
+		FFmpegPath: "ffmpeg",
+	})
+	if !sliceContainsPair(args, "-hwaccel", "cuda") {
+		t.Error("NVENC must request -hwaccel cuda")
+	}
+	if !sliceContainsPair(args, "-c:v", "h264_nvenc") {
+		t.Error("NVENC must use h264_nvenc encoder")
+	}
+	if sliceContains(args, "-preset") {
+		// HW encoders ignore software preset; we should NOT pass it.
+		t.Error("HW encoder path should not include -preset")
+	}
+}
+
+func TestBuildFFmpegArgsAddsStartSeek(t *testing.T) {
+	args := buildFFmpegArgs("/tmp/movie.mp4", TranscodeOpts{
+		Action:       ActionPassthrough,
+		StartSeconds: 90.5,
+		FFmpegPath:   "ffmpeg",
+	})
+	idxSs, idxIn := -1, -1
+	for i, a := range args {
+		if a == "-ss" {
+			idxSs = i
+		}
+		if a == "-i" {
+			idxIn = i
+		}
+	}
+	if idxSs < 0 {
+		t.Fatal("missing -ss flag")
+	}
+	if idxIn < 0 {
+		t.Fatal("missing -i flag")
+	}
+	if idxSs >= idxIn {
+		t.Errorf("expected -ss BEFORE -i for fast seek; got -ss@%d -i@%d", idxSs, idxIn)
+	}
+	if args[idxSs+1] != "90.500" {
+		t.Errorf("expected seek 90.500s, got %q", args[idxSs+1])
+	}
+}
+
+func TestBuildFFmpegArgsDownscale(t *testing.T) {
+	args := buildFFmpegArgs("/tmp/movie.mkv", TranscodeOpts{
+		Action:     ActionTranscodeVideo,
+		HWAccel:    HWAccelNone,
+		MaxHeight:  720,
+		FFmpegPath: "ffmpeg",
+	})
+	hasVF := false
+	for i := 0; i < len(args)-1; i++ {
+		if args[i] == "-vf" && strings.Contains(args[i+1], "720") {
+			hasVF = true
+		}
+	}
+	if !hasVF {
+		t.Errorf("expected -vf scale containing 720; args=%v", args)
+	}
+}