Replace the AVC-Intra growing path (which Premiere rejected as unsupported/
damaged) with VC-3/DNxHD written directly by ffmpeg native MXF muxer. The
frame-wrapped OP1a body grows readably mid-write and imports+grows live in
Adobe Premiere (matches vMix workflow). No raw2bmx, no FIFO orchestrator, no
footer-finalize ordering - one ffmpeg writes MXF straight to the SMB share.
Two operator-selectable bitrates: vc3_90 (~90 Mbps, default) and vc3_220
(~220 Mbps). Both 8-bit 4:2:2 @ 1080p59.94, essence VC3_1080p_1238. Stop uses
a plain SIGINT (ffmpeg flushes the MXF footer cleanly).
UI: growing codec select (90/220) replaces the AVC-Intra readonly field; the
freeform growing bitrate input is removed (bitrate is codec-fixed). mam-api
guards accept vc3_90/vc3_220, default vc3_90.
Verified on zampp3: both bitrates grow live + finalize clean (check-complete
passes, 0 decode errors), user-confirmed Premiere import + growth.
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