stem-worker: byte-exact pad parity — the bar-seam double-render is gone

The pads diverged (corr 0.94) because the old worker rendered each bar in isolation: bar N's final block (the s-BS declick pre-roll of bar N+1's downbeat) was rendered once with STALE params, then double-rendered as bar N+1's discarded pre-roll

• Fix: split the walk into ingest (queue a bar's events) and render (windowed block walk), driven as a one-bar pipeline

• Parity now BYTE-EQUAL (corr 1.000000, maxΔ 0) on every bus across four states incl. added cached-dx7-lead and cached-supersaw-lead cases. render-core/press untouched; engine.test ALL PASS. ~3.4x realtime.

▶ Play it: Mercury Interrogation — where the harmony organ has the least anywhere to hide https://stellate.app/?genre=bebop&seed=9330&path=694.13834,713.13867,675.13867&xdur=13.122

0dc9590 https://github.com/aboard-io/stellate/commit/0dc95902c45322ea2f5b6a301a47fe52174fd6e3