Web: metadata callbacks are declared but never invoked — wire Shaka timed metadata and chapters

[puckey]

What

src/web/NativeAudioBrowser.ts:160-162 declares three metadata callbacks as no-op defaults:

onChapterMetadata: (chapters: ChapterMetadata[]) => void = () => {}
onTrackMetadata: (metadata: TrackMetadata) => void = () => {}
onTimedMetadata: (metadata: TimedMetadata) => void = () => {}

Nothing in src/web/ ever calls them. src/features/metadata.ts documents all three unconditionally, so a consumer targeting web subscribes to an emitter that can never fire, with nothing to indicate that. The now-playing formatter's timedMetadata parameter is undefined on web for the same reason.

What's feasible

The web player is Shaka 4.x (src/web/TrackPlayer/Player.ts), attached to an <audio> element:

• Timed metadata (HLS ID3 / DASH EMSG) — Shaka dispatches a metadata event. Map it to onTimedMetadata. (Present in the shipped bundle; the typed .d.ts has no typed event map for it, so confirm the payload shape against Shaka's docs when implementing.)
• Chapters — player.getChapters(language) / getChaptersAsync(), plus Shaka's handling of chapters text tracks. Map to onChapterMetadata.
• Static track metadata — no clean Shaka equivalent. ID3 frames observed at load could approximate it, but the semantics wouldn't match the native side.

What's not feasible

ICY (Icecast/Shoutcast) metadata on progressive HTTP streams. Those play through the browser's own pipeline, which strips the interleaved metadata and exposes no API for it. Supporting it would mean:

1. fetching the stream with Icy-MetaData: 1;
2. reading icy-metaint, which requires the origin to send Access-Control-Expose-Headers: icy-metaint;
3. de-interleaving metadata blocks from the audio byte stream;
4. feeding the remaining audio through MSE — ManagedMediaSource on iOS Safari.

That's a substantial subsystem whose success depends on per-origin CORS cooperation the library can't require. For consumers whose sources are predominantly ICY, this issue buys little — worth weighing before picking it up.

Proposal

1. Wire onTimedMetadata and onChapterMetadata from Shaka.
2. Add @platform documentation to all three, stating what does and doesn't arrive on web — specifically that ICY timed metadata never does.
3. Leave onTrackMetadata inert but documented.

Verification

Needs an HLS stream carrying ID3 timed metadata; there's no such fixture in the repo today.

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Relates to #97 (task 4).