feat(stream): GPU HDR tonemap via libplacebo

Prefer the single-pass Vulkan libplacebo filter over the CPU zscale chain
for HDR->SDR tonemapping when the agent ffmpeg has it. One GPU pass does
tonemap + BT.709 primaries/transfer/matrix + 8-bit yuv420p, replacing the
four-stage zscale chain and its trailing format=/setparams. Higher quality,
far cheaper than the CPU path, and present on builds that lack zscale.

- FFmpegSupportsLibplacebo probe (cached, mirrors FFmpegSupportsZscale)
- HasLibplacebo on TranscodeRuntime, wired from buildTranscodeRuntime
- hls.go: videoTail picks libplacebo when present (not h264_vaapi), else
  keeps the zscale tonemap + format chain
- test: libplacebo replaces the zscale chain, never runs alongside it

internal/engine/tonemap.go

@@ -28,11 +28,53 @@ import (
 // possible follow-up if HLG/DV-only sources become common.
 const hdrTonemapChain = "zscale=t=linear:npl=100,format=gbrpf32le,zscale=p=bt709,tonemap=tonemap=hable:desat=0,zscale=t=bt709:m=bt709:r=tv,"
 
+// libplaceboTonemapFilter maps an HDR source to SDR BT.709 in a SINGLE GPU pass
+// (Vulkan): tone-map the HDR curve, convert primaries/transfer/matrix to BT.709
+// limited range, and output 8-bit yuv420p — so it REPLACES the zscale chain AND
+// the trailing `format=yuv420p,setparams=bt709` (it does both). Higher quality
+// and far cheaper than the CPU zscale chain, and the agent's ffmpeg has it where
+// zscale is missing. It does NOT scale here — the CPU scale chain runs first
+// (it owns the even-dimension rounding libx264/nvenc require). No trailing comma:
+// it's the last filter in the chain.
+const libplaceboTonemapFilter = "libplacebo=colorspace=bt709:color_primaries=bt709:color_trc=bt709:range=tv:format=yuv420p:tonemapping=bt.2390"
+
 var (
 	zscaleCacheMu sync.Mutex
 	zscaleCache   = map[string]bool{}
+
+	libplaceboCacheMu sync.Mutex
+	libplaceboCache   = map[string]bool{}
 )
 
+// FFmpegSupportsLibplacebo reports whether the ffmpeg binary has the libplacebo
+// filter (Vulkan GPU HDR tonemap + colorspace). Preferred over zscale when both
+// exist. Cached per path; a probe failure is treated as "no". Mirrors
+// FFmpegSupportsZscale.
+func FFmpegSupportsLibplacebo(ffmpegPath string) bool {
+	if ffmpegPath == "" {
+		return false
+	}
+	libplaceboCacheMu.Lock()
+	if v, ok := libplaceboCache[ffmpegPath]; ok {
+		libplaceboCacheMu.Unlock()
+		return v
+	}
+	libplaceboCacheMu.Unlock()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	out, err := exec.CommandContext(ctx, ffmpegPath, "-hide_banner", "-filters").Output()
+	supported := err == nil && bytes.Contains(out, []byte("libplacebo"))
+
+	libplaceboCacheMu.Lock()
+	libplaceboCache[ffmpegPath] = supported
+	libplaceboCacheMu.Unlock()
+	if supported {
+		log.Printf("[tonemap] ffmpeg has libplacebo — HDR sources tonemapped on the GPU (preferred)")
+	}
+	return supported
+}
+
 // FFmpegSupportsZscale reports whether the ffmpeg binary at path was built with
 // the zscale filter (libzimg), required for HDR→SDR tonemapping. Cached per
 // path. A detection failure (binary missing, exec error) is treated as "no" so