Add SFX transition channel (risers, sweeps, impacts, reverse, noise)

New instr 20 with 6 transition types (riser / sweep / downlift / impact / reverse swell / noise hit), one per section as an entering transition: lead-in types end on the section downbeat, hits land on it.

• Engine verified: renders clean with SFX (peak 0.71, 0 errors). UI verified headless: 6 instrument columns / 48 cells, mobile no overflow, no errors.

▶ Play it: Food Court Eternity — the brightest, most processed corner of the space https://stellate.app/?genre=vaporwave&seed=3628&path=1679.4393,1698.4426,1660.4426&xdur=13.122

e194934 https://github.com/aboard-io/stellate/commit/e19493446c6ce5d97a32bcbb52f790cfd96c5e9a