Function ffmpeg_available 

pub fn ffmpeg_available() -> bool

Returns true if ffmpeg is on PATH.

Single source of truth for “should we even attempt to shell out to ffmpeg” — extract_video_frame below already surfaces a missing binary as RustmotionError::FfmpegSpawn on first use, but callers that decode many frames up front (preload::preextract_video_frames) want to check once and print one clear warning instead of failing identically once per frame. Mirrors the ffmpeg_available helper the rustmotion crate’s encode::video_audio module already uses for the embedded-audio extraction path (PR #151) — same check, same reasoning, different asset kind.