Explorer overhaul: dynamic genre layout, voice timeline, unified background chip

Genre star map is now laid out DYNAMICALLY at load (deterministic force-directed relaxation with label-box repulsion) instead of a hand-tuned POS table

• The busted mixer (its gain/mute/solo were no-ops on the WAV engine) is gone, replaced by an "INSIDE THE SOUND" info tool: live genre-blend, a feel radar, and a per-voice TIMELINE piano-roll showing each…

• Chips slimmed (the play button was overflowing off-screen): note chip removed, video + demoscene merged into one background chip (off → video → demoscene), default off

Verified: explorer-ui-test (93 stars, 0 label overlaps, loop/zoom/traveler), genre-viz-test (timeline, note-feed, mixer-gone).

▶ Play it: Polyrhythm Senate — parked on the genre this change names https://stellate.app/?genre=afrobeat&seed=3544&path=1237.17797,1256.17830,1218.17830&xdur=13.122

b940958 https://github.com/aboard-io/stellate/commit/b94095887a7c37a0d01d26aeba1fe20b029cacbf