Stage 3 scaffolding: CBEATS chord-bar refactor + stem classing + dormant worker (WIP)

The verified, inert groundwork for the 16-bar stem cache:

• state-engine stemClass(units): CACHED = unitCost >= 2.0 (dx7 fleet, supersaw leads, heavy pads + their inserts — R7's steady-state cost); LIVE = bass/drums/stab/sfx/vocoder/sampler/mono-legato.

• live.js CBEATS refactor: the three hardcoded `8`s (bar window, beatAbs, nextTime advance) now route through Math.max(2,round(chordEvery||8)), matching csd-engine buildEvents

• LOOKAHEAD widens to ~10s only under opts.stems; `stem` handle is null-safe and dormant. onBar's stemRepeat flag is scaffolding.

• faust/stem-worker.js + stem-parity-test.js: WIP — worker renders CACHED units through render-core with persistent per-unit offline processors

Verified: green: engine.test ALL PASS, validate-genres --quick PASS, matrix untouched.

▶ Play it: Atrium Standing Wave — parked on the genre this change names https://stellate.app/?genre=mallsoft&seed=9331&path=1873.8805,1892.8838,1854.8838&xdur=13.122

9667165 https://github.com/aboard-io/stellate/commit/9667165bcdf905d04cd75689e133d6ff08d98531