F.3: make the whole-path AUDIO export walk async + chunked

buildLoopPlan walked the entire loop SYNCHRONOUSLY on the main thread (same shape as the MIDI-export crash) — a long path's per-bar stateAt + buildEvents + voice units froze the page.

Verified: journey.js/export.js parse; simulate-path gate green (walk intact). Browser export-flow verification to follow at the checkpoint.

▶ Play it: Food Court Eternity → Smoke Arithmetic → Barometric Stampede — a constellation across three families — the path is the mix https://stellate.app/?genre=vaporwave&seed=6521&path=1679.4415,292.16891,1718.20526&xdur=0.046

17ab74c https://github.com/aboard-io/stellate/commit/17ab74c91b9cc83c2d7e82f55e07f4ade129f1a0