Client-side export via WebCodecs + Mediabunny

[ronak-create]

What

Move the fast-export encode into the browser using WebCodecs for encoding and Mediabunny for muxing.

Suggested by Mediabunny's author (vanilagy) in the Show HN thread.

Why

The current fast-export path renders each frame on canvas, encodes it as JPEG, POSTs it to the server, and ffmpeg encodes the MP4. That means:

• a lossy JPEG intermediate for every frame
• an HTTP round trip per frame
• ffmpeg on PATH is required for high-quality export (biggest setup friction we have)

With WebCodecs + Mediabunny the canvas frames go straight into a (usually hardware) encoder and get muxed to MP4 client-side. No JPEG hop, no server round trips, no ffmpeg requirement for export. It also unblocks #hosted-demo-mode, since export would work on a static host.

Tradeoffs to settle

• This would be the first dependency in a zero-dependency repo. Proposal: vendor Mediabunny as a single file under vendor/ so node server.js stays the only install step.
• Keep the ffmpeg path as a fallback (it still handles upload remuxing and the analyze pipeline regardless).
• Audio: the offline WAV mix needs an AudioEncoder pass (AAC or Opus) instead of ffmpeg's encode.

Done when

Export produces an MP4 of comparable quality to the current CRF-18 path with ffmpeg absent from PATH, in Chromium.

[PlkMarudny]

WebCodecs, however, when using a hardware encoder offer limited options of encoding modes, so constant bitrate media will be produced.