Mobile background audio: media-element output + lock-screen controls

Paul: keep audio playing when the screen blanks. A bare WebAudio graph is silenced on mobile screen-lock; a PLAYING media element is kept alive as media playback.

• Media Session: lock screen shows the current genre (or "a + b" when the second blend weight rides above 25%, mirroring the chyron) by CONSTELLATE, updated on genre change via onBar

Verified: live-test-run PASS through the element route (load 0.988, 0 errors); dedicated headless verify

▶ Play it: Food Court Eternity — the brightest, most processed corner of the space https://stellate.app/?genre=vaporwave&seed=6495&path=1679.4393,1698.4426,1660.4426&xdur=13.122

281d013 https://github.com/aboard-io/stellate/commit/281d013799e697e52819d4dfebe785b1cc02e66d