Builds ffmpeg arguments for the stream-copied video continuation of an encode-then-copy
plan. Output is MPEG-TS with rebased timestamps (no -copyts — preserving source PTS in
an MP4 mux produces an empty file when the seek point is deep into a long clip).
Parameters
inputPath: string
Source media path.
outputPath: string
Copy segment .ts output path.
copyStart: number
Start of the copy segment — a known keyframe timestamp.
Builds ffmpeg arguments for the stream-copied video continuation of an
encode-then-copyplan. Output is MPEG-TS with rebased timestamps (no-copyts— preserving source PTS in an MP4 mux produces an empty file when the seek point is deep into a long clip).