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/transcode_quality.go

@@ -18,6 +18,10 @@ type TranscodeRuntime struct {
 	// Set only when the ffmpeg build has zscale (FFmpegSupportsZscale); without
 	// it the tonemap filter would error and break playback, so it stays off.
 	TonemapHDR bool
+	// HasLibplacebo: the ffmpeg build has the libplacebo filter (GPU HDR tonemap).
+	// Preferred over the zscale chain for HDR sources — one GPU pass, higher
+	// quality, and present where zscale is missing.
+	HasLibplacebo bool
 }
 
 // qualityCap maps a session's Quality label to a (MaxHeight, VideoBitrate)