fix(growing): drop FIFO FD-priming deadlock — ffmpeg-first ordering feeds raw2bmx frames

Root cause of frozen 22KB growing MXF: the parent held a stray read-write
priming writer (FD 7) on the video FIFO while raw2bmx opened it, so raw2bmx
read a malformed/empty stream start and exited after the header (Duration:0).
Proven live on zampp3: removing the FD-7/8 priming + fd-watch loop and simply
starting ffmpeg before raw2bmx lets the blocking FIFO opens pair up naturally
(ffmpeg sole writer). True-1080p59.94 AVC-Intra 100 then grows monotonically
on disk and a mid-write snapshot decodes to its last frame (481 frames @ 8s).

services/capture/src/capture-manager.js

@@ -937,50 +937,26 @@ const bmx = [
     // overwrites them in-place. It is killed by the cleanup trap on exit.
     const script = `
 set -u
-# Save the ORIGINAL stdin (the fc_pipe video stream Node pipes in) to FD 9
-# BEFORE any FIFO priming touches fd 0. ffmpeg later reads from FD 9 explicitly,
-# guaranteeing it is the sole reader of the raw video — running ffmpeg as a
-# backgrounded subshell that merely inherited fd 0 starved it of bytes when the
-# input was a pipe (the "No filtered frames / empty output" growing failure).
 exec 9<&0
 VF=$(mktemp -u /tmp/grow_v.XXXXXX); AF=$(mktemp -u /tmp/grow_a.XXXXXX)
 OUT=${sh(outPath)}
 mkfifo "$VF" "$AF"
-PATCHPID=
-cleanup() { rm -f "$VF" "$AF"; [ -n "$PATCHPID" ] && kill "$PATCHPID" 2>/dev/null; }
- trap cleanup EXIT
-# Prime both FIFOs read-write (non-blocking) to break the open-order deadlock.
-exec 7<>"$VF" 8<>"$AF"
-# raw2bmx: close priming FDs (no stray writer) + read stdin from /dev/null before
-# exec so it sees real EOF and never competes for the video pipe.
-( exec 7>&- 8>&- 9>&- 0</dev/null; exec ${bmxLine} >/tmp/raw2bmx.log 2>&1 ) &
-BMXPID=$!
-# ffmpeg reads the raw video from FD 9 (the saved original stdin). pipe:0 in its
-# arg list maps to fd 0, so we point fd 0 at FD 9 for the child.
-( exec 7>&- 8>&- 0<&9 9<&-; exec ${ffLine} ) &
+cleanup() { rm -f "$VF" "$AF"; }
+trap cleanup EXIT
+( exec 0<&9 9<&-; exec ${ffLine} ) &
 FFPID=$!
-# Parent no longer needs the saved stdin — drop it so ffmpeg is the sole reader.
 exec 9<&-
-# Forward a clean stop to ffmpeg; raw2bmx then gets EOF and finalizes the footer.
+exec ${bmxLine} >/tmp/raw2bmx.log 2>&1 &
+BMXPID=$!
 stop() { kill -INT "$FFPID" 2>/dev/null; }
 trap stop INT TERM
-# Drop the parent priming FDs once raw2bmx has opened BOTH FIFOs, so ffmpeg is
-# the sole writer (its EOF reaches raw2bmx). If raw2bmx dies early, bail.
-for i in $(seq 1 200); do
-  kill -0 "$BMXPID" 2>/dev/null || break
-  n=$(ls -l /proc/$BMXPID/fd 2>/dev/null | grep -c -- "$VF\\|$AF")
-  [ "\${n:-0}" -ge 2 ] && break
-  sleep 0.1
-done
-exec 7>&- 8>&-
-PATCHPID=
-# Wait for ffmpeg (source end), then for raw2bmx to finalize the footer.
 wait "$FFPID"; FFRC=$?
 wait "$BMXPID"; BMXRC=$?
 echo "[grow] ffmpeg rc=$FFRC raw2bmx rc=$BMXRC out=$OUT" >&2
 exit "$BMXRC"
 `;
     return ['-c', script];
+    return ['-c', script];
   }
 
   /**