Compress the export: WebCodecs in the browser, muxing here

[Gamah]

Closes PLAN row 99.

songToWav re-rendered the song and wrapped it raw, so one save was ~13 MB of 16-bit stereo. The web export now encodes in the browser.

What it does

• web/encode.js (new, DOM-free): probes AudioEncoder.isConfigSupported once per realm and picks FLAC if the browser encodes it, else Opus at 256 kb/s (application: audio, signal: music, complexity 10). Neither → the engine's WAV, so the button always does something. The probe also labels the button (⬇ .flac / ⬇ .opus / ⬇ .wav), rather than promising an extension the browser may not produce.
• WebCodecs returns encoded chunks and no container, so the muxing is ours: FLAC is description (fLaC + STREAMINFO) followed by the frames; Opus gets a real page-by-page Ogg stream — OpusHead/OpusTags, lacing, granule positions at 48 kHz starting at the pre-skip, and Ogg's non-reflected CRC. The resolved seed rides along as a TITLE tag.
• The engine renders at 44.1 kHz and an encoder may refuse that, so there is a 48 kHz fallback through OfflineAudioContext rather than a hand-rolled resampler.
• Export now reuses the PCM already in the cache when the song is one the timeline is holding, instead of re-rendering it.

Web-only by choice: Engine/ is untouched, s&box keeps writing WAV through Wav.cs, and no wasm re-stage is involved (tools/bundle-stamp.sh check is clean).

Drive-by fix

The playlist's per-row seek and export passed row.n — the song's index in its station, which under shuffle is 0 for every row. Both take row.position now.

Testing

• test/encode.mjs (new, in make test) demuxes the Ogg stream back out page by page against the framing spec: CRC checked against an independent bit-at-a-time implementation, headers, lacing (including the 255-multiple 0 terminator), the 255-segment page limit, packets byte-for-byte, granule monotonicity and pre-skip. FLAC's missing/bogus header is an error rather than a guess.
• test/{queue,palette,player,element}.mjs and test/{smoke,page}.mjs pass; the single-file bundler was rebuilt and test/dist-single.mjs passes.
• Not tested here: any actual encode. WebCodecs is the browser's and there is no node build of it, and no engine or browser toolchain on this host — the FLAC path in particular is untestable until a browser ships FLAC encoding. Needs an ear and a real browser.

Spec citations in web/encode.js are dated 2026-08-13 (WebCodecs + its FLAC/Opus registrations, RFC 7845, Ogg framing).