Builds libx264 arguments shared by every smart-cut encode (head segments and
whole-selection encodes for cuts that never leave a single GOP).
A large GOP + no B-frames + disabled scene-cut detection guarantees the encoded
segment contains exactly one keyframe (its first frame), which is what makes it
safe to concatenate with a stream-copied continuation.
Parameters
frameRateRational: string
Target output frame rate as an ffprobe-style rational.
Builds libx264 arguments shared by every smart-cut encode (head segments and whole-selection encodes for cuts that never leave a single GOP). A large GOP + no B-frames + disabled scene-cut detection guarantees the encoded segment contains exactly one keyframe (its first frame), which is what makes it safe to concatenate with a stream-copied continuation.