Phase 4 Step 9: Video/audio media elements

[thomasnemer]

Parent: #41

Goal

Implement <video> and <audio> media elements with playback support, prioritizing patent-free AV1+Opus in WebM.

Prerequisites

• Step 7 (Shadow DOM) — default media controls are rendered via shadow DOM

Implementation

• HTMLMediaElement API in JS: play(), pause(), currentTime, duration, volume, muted, src, media events (play, pause, ended, timeupdate, etc.)
• Primary format: AV1+Opus in WebM container (patent-free, privacy-respecting)
• Audio pipeline: symphonia for decoding, cpal for audio output
• Video pipeline: dav1d for AV1 decoding, frames uploaded to wgpu texture, double-buffered
• AV sync: synchronize audio and video streams using presentation timestamps
• Default media controls rendered via shadow DOM (play/pause button, seek bar, volume, fullscreen)
• New crate: ie-media
• New dependencies: symphonia, dav1d, cpal

Tests

• Media element API tests (play, pause, seek, volume)
• Audio decoding and playback tests
• Video frame decoding and texture upload tests
• AV sync tests
• Media event firing tests
• Default controls interaction tests

Acceptance Criteria

• <video> and <audio> elements play AV1+Opus WebM content
• HTMLMediaElement JS API works correctly
• Audio outputs through system audio via cpal
• Video frames render to wgpu texture with double buffering
• Default media controls are functional via shadow DOM