fix(stream): functional libplacebo probe + benchmark hardening

Review (critico) caught a regression: the prod agent image ships a BtbN GPL
ffmpeg with libplacebo COMPILED IN but no Vulkan runtime (debian-slim, no
libvulkan1/mesa-vulkan-drivers/nvidia ICD). The presence probe (ffmpeg
-filters) would flip HasLibplacebo on, the filter's Vulkan device creation
would fail at runtime, and HDR sources that previously tonemapped via zscale
would break.

- FFmpegSupportsLibplacebo now RUNS the filter on one synthetic frame and
  requires a clean exit (forces Vulkan device init + filtergraph negotiation),
  so it is honest about THIS host: works on Vulkan-capable hosts, falls back to
  zscale where Vulkan is absent. Logs the real ffmpeg error on failure.
- Warm the libplacebo (Vulkan init ~1.7s) + zscale caches in a background
  goroutine at startup so the first stream session doesn't pay the probe and
  risk its setup timeout.
- Benchmark: margin 1.5x -> 2.0x (the probe measures encode only; real decode
  of HEVC/10-bit + busier content needs more headroom), per-probe timeout
  12s -> 6s + overall 45s -> 20s (it blocks registration on software hosts),
  and a 'no rung measured' case (missing lavfi/wedged ffmpeg) now keeps the
  1080 default instead of flooring at 480 — an infra failure isn't a slow host.

Verified e2e on the fixed binary: LOTR Two Towers (HEVC 3840x1608 10-bit
HDR10/PQ, 12GB) on desktop-Chrome caps -> hls, ffmpeg runs h264_nvenc with
-vf ...,libplacebo=...:format=yuv420p:tonemapping=bt.2390 (zscale chain
replaced), 45 fMP4 segments, ffprobe confirms output h264 yuv420p bt709
(tonemapped from bt2020/smpte2084), no ffmpeg errors.

internal/cmd/daemon.go

@@ -160,12 +160,27 @@ func runDaemonStart() error {
 	// HW encoders return 2160 instantly; a software-only host runs a bounded
 	// encode benchmark so a weak NAS/CPU reports the rung it can actually
 	// sustain (720/480) and the web side routes oversized sources to an
-	// external player instead of a stuttering transcode. Own timeout — the 10 s
-	// probeCtx above is sized for the quick diagnostic, not three encode rungs.
-	benchCtx, benchCancel := context.WithTimeout(context.Background(), 45*time.Second)
+	// external player instead of a stuttering transcode. This blocks
+	// registration on a software host, so it's bounded tight (3 rungs × 6 s =
+	// 18 s worst case; <1 s on a capable box that passes the first rung). Own
+	// timeout — the 10 s probeCtx above is sized for the quick diagnostic.
+	benchCtx, benchCancel := context.WithTimeout(context.Background(), 20*time.Second)
 	maxTranscodeHeight := engine.BenchmarkMaxTranscodeHeight(benchCtx, ffmpegResolved, hwAccelPick)
 	benchCancel()
 
+	// Warm the tonemap capability caches off the hot path. The libplacebo probe
+	// actually RUNS the filter (Vulkan device init ~1.7 s), so doing it lazily
+	// in buildTranscodeRuntime would tax the FIRST stream session and risk its
+	// setup timeout. A real session arrives seconds-to-minutes after startup, so
+	// a background warm has finished by then; if one races in first, the cache's
+	// own mutex makes the concurrent cold call safe (both compute the same bool).
+	if cfg.Download.Transcode.Enabled && ffmpegResolved != "" {
+		go func() {
+			engine.FFmpegSupportsLibplacebo(ffmpegResolved)
+			engine.FFmpegSupportsZscale(ffmpegResolved)
+		}()
+	}
+
 	// Create daemon config
 	daemonCfg := agent.DaemonConfig{
 		AgentID:            cfg.Agent.ID,