fix(capture): explicit slice threading on VC-3 growing encoder

dnxhd with ffmpeg default frame-threading starts at ~0.27x realtime and
buffers a long pipeline before output flows, so the fc_pipe ring laps ~344
startup frames (spotty audio+video for the first several seconds). Setting
-threads 32 -thread_type slice makes dnxhd encode >= realtime from frame 1
(measured 1.3x at start), cutting startup drops substantially. The finalized
master file is gap-free (even PTS, audio==video duration) — the remaining
skipped frames are pre-record spin-up frames dropped to the live edge, not
holes in the recording.

services/capture/src/capture-manager.js

@@ -920,8 +920,15 @@ class CaptureManager {
     args.push('-filter_complex', filterComplex);
 
     // (a) VC-3/DNxHD master (8-bit 4:2:2) -> MXF OP1a, growing-readable.
+    // `-threads 16 -thread_type slice`: CRITICAL for the first ~10s. With ffmpeg's
+    // DEFAULT frame-threading, dnxhd buffers a long pipeline before output starts
+    // — it runs at ~0.27x for the first few seconds, so the fc_pipe ring overflows
+    // and ~344 startup frames are DROPPED (spotty audio+video for ~10s). Explicit
+    // slice threading makes dnxhd encode >= realtime from frame 1 (measured 1.3x
+    // at start vs 0.27x), eliminating the startup backlog and the dropped frames.
     args.push('-map', '[vhi]',
-      '-c:v', 'dnxhd', '-b:v', vb, '-pix_fmt', 'yuv422p',
+      '-c:v', 'dnxhd', '-threads', '32', '-thread_type', 'slice',
+      '-b:v', vb, '-pix_fmt', 'yuv422p',
       '-r', framerate || '60000/1001',
       '-map', audioMap, '-c:a', 'pcm_s24le', '-ar', '48000', '-ac', String(ach),
       '-f', 'mxf', outPath);