Fold SFX into the ⚡ transition control; drop the SFX column

Per feedback, the lightning-bolt is now a per-section transition menu instead of a drum-fill toggle: off / drum fill / riser / sweep / downlift / impact / reverse / noise.

• Engine: sec.fill (string) drives the end-of-section transition — "drum fill" emits the snare roll; SFX types emit instr 20 (build-ups in the final bar, hits on the next downbeat).

• Defaults: pre-chorus -> riser, bridge -> drum fill.

• Verifier counts any transition; improve adds a riser into the drop.

• Fix: ⚡ menu was self-closing (click bubbled to the popover's outside-click handler) — stopPropagation.

Verified: engine renders clean; ⚡ menu lists all 8 transitions.

▶ Play it: Barometric Stampede — the genre that is mostly drums, so the kit has nowhere to hide https://stellate.app/?genre=jungle&seed=9620&path=1718.20504,1737.20537,1699.20537&xdur=13.122

40bb15b https://github.com/aboard-io/stellate/commit/40bb15bcc5813389ed873b02da56f5413e649893