Kill the stem clicks at the root: crossfade seams + eliminate resets

THE root cause of the travel clicks, proven with the Faust click monitor + CPU throttling: the stem cache butt-spliced consecutive per-bar buffers, relying on the worker's persistent-processor continuity for sample-clean seams.

• Fix 1 — equal-power crossfade at every NON-continuous seam: on a reset/vamp/fallback the incoming bar starts ~6ms early and its head fades in (cos/sin) while the outgoing tail fades out, turning the step into…

• deterministic fixed path, stems-ON vs stems-OFF: hard-jumps(>0.9) 0 vs 0, totals ~equal (program content only) — cache adds no clicks.

• FORCED 10 resets mid-travel: 0 hard jumps, 0 near any reset — the crossfade masks the exact discontinuity that used to click.

• natural resets 0/run (was 2-5). Transparency: stationary corr 0.9994-1.0. engine.test ALL PASS; stem-parity byte-exact (worker untouched); node --check clean.

Verified: (playwright + 4× CPU throttle, Faust clickmon):

▶ Play it: Highway Heat Death — where the synthesis is the identity, not the decoration https://stellate.app/?genre=synthwave&seed=4148&path=859.9909,878.9942,840.9942&xdur=13.122

e1f8b23 https://github.com/aboard-io/stellate/commit/e1f8b234243458caf182a1e313864ba7f2219058