Fix MIDI-still-crashing: async chunked whole-path walk (don't freeze the page)

The 2048-bar cap wasn't enough: the per-bar stateAt (K.mix genre blend) + buildEvents cost ~11ms, so a long loop's SYNCHRONOUS main-thread walk blocked the UI ~22s and the browser killed the page.

▶ Play it: Highway Heat Death — where the synthesis is the identity, not the decoration https://stellate.app/?genre=synthwave&seed=6618&path=859.9909,878.9942,840.9942&xdur=13.122

d62db0d https://github.com/aboard-io/stellate/commit/d62db0da89cd76ee874e61afce6583a36f49a44a