feat(transcode): tonemap HDR sources to SDR (zscale-gated)

HDR (HDR10/HLG/Dolby Vision) transcoded to SDR came out washed-out and
desaturated because the filter chain never tonemapped. buildHLSFFmpegArgsAt now
inserts a zscale linearise -> hable tonemap -> BT.709 chain after the scale and
before format=, but only when the source is HDR and the ffmpeg build has zscale
(FFmpegSupportsZscale, cached). Builds without zimg keep the old behaviour
(plays, just desaturated) instead of erroring.

It's a CPU filter, valid for every encoder here: the decode hwaccel deliberately
leaves frames in system memory (no -hwaccel_output_format), so zscale runs ahead
of format=/hwupload exactly like the existing scale filter. Verified on a real
4K HDR10 file — vivid colour and deep blacks vs the washed-out baseline.

internal/cmd/player_session_registry.go

@@ -92,5 +92,8 @@ func buildTranscodeRuntime(ctx context.Context, cfg config.Config) engine.Transc
 		VideoBitrate: cfg.Download.Transcode.VideoBitrate,
 		AudioBitrate: cfg.Download.Transcode.AudioBitrate,
 		MaxHeight:    cfg.Download.Transcode.MaxHeight,
+		// Tonemap HDR→SDR only when this ffmpeg build has zscale; otherwise the
+		// filter would error and break playback, so HDR plays untonemapped.
+		TonemapHDR: engine.FFmpegSupportsZscale(ffmpegPath),
 	}
 }