Function transcode_to_webm 

pub fn transcode_to_webm(input: &Path, output: &Path) -> Result<(), EncodeError>

Transcode an existing video file (the MP4/H.264 recording scratch) into a VP9 + Opus .webm at output. Drives the Save panel’s “WebM” export: the recorder always captures to an MP4/H.264 scratch (the canonical intermediate), and a WebM export re-encodes it here.

Probes input for an audio track via scratch_has_audio and includes the Opus leg only when one is present — a screen-only recording’s scratch has no audio track, and wiring an audio branch to a decodebin pad that never appears would hang webmmux waiting for EOS on it.

VP9 has no Apple HW encoder, so this uses the vp9enc software encoder (same as the live WebM encode path) — a short clip takes a few seconds. Call it off the main thread (the recorder runs it via spawn_blocking).

Errors

• EncodeError::InvalidConfig — input doesn’t exist.
• EncodeError::Spawn — gst-launch-1.0 missing from PATH.
• EncodeError::PipelineFailed — the transcode exited non-zero.