Video export: fix silent canvas taint + iOS guard + container/ext + hidden-tab

docs/ROADMAP.md 2.1-2.3 statically-sound fixes:

• restrict the export take to LOCAL clips (a cross-origin drawImage silently taints the canvas; the old try/catch could not catch it) + fix the false "caught and skipped" header comment

• feature-detect captureStream + a working audio track; disable on iOS/unsupported

• derive the download extension AND Blob type from rec.mimeType (was hard-coded .webm)

• refuse/pause recording when document.hidden (drop the false "immune to throttling")

Verified: video-export-probe PASS (168 frames, audio, graceful taint-degrade, hidden-tab refusal). Safari-mp4 / iOS-disable / real-remote-taint / real-throttle remain device-pending.

▶ Play it: Highway Heat Death — where the synthesis is the identity, not the decoration https://stellate.app/?genre=synthwave&seed=6642&path=859.9909,878.9942,840.9942&xdur=13.122

dc5dc84 https://github.com/aboard-io/stellate/commit/dc5dc84adab8ff9cefc4e56de2219587cb836f89