feat(core): expose frame-rate variables to presets

Add input_frame_rate, input_fps, and output_gop_size to the set of
variables available to preset files at source time.

input_frame_rate is the raw ffprobe fraction (e.g. 30000/1001).
input_fps rounds that fraction to the nearest integer via awk.
output_gop_size is derived as roughly five seconds of video (input_fps *
5), capped at 300 frames.

The ffprobe invocation is extended to request r_frame_rate alongside the
existing codec_name and pix_fmt entries; a missing frame rate is now
treated as a hard error. Dry-run and verbose output are updated to
display the new values. Documentation (header comment, --help text, man
page, README) is updated accordingly.

0 files changed, 0 insertions, 0 deletions