Voices breathe: musical dynamics in the composition

Paul: "voices today enter and exit abruptly. Very often dynamics are an active part of every composition." They were right — a section turning a voice on snapped it to full level; turning it off cut it dead.

• Add a renderer-stage dynamics pass (applyVoiceDynamics). A harmonic/melodic voice — pad, bass, melody

• Serves BOTH engines. Press hands the whole multi-section song, so the run edges are computed from state.sections + spans.

• Renderer-only — the symbolic verifier and musicality never see it, so the confusion matrix is untouched.

• (stem-parity's citypop_s7 error is pre-existing — identical before/after, the sampled-by-default re-pin already filed in NEXT.md.)

▶ Play it: Mercury Interrogation — where the harmony organ has the least anywhere to hide https://stellate.app/?genre=bebop&seed=7229&path=694.13834,713.13867,675.13867&xdur=13.122

14daa06 https://github.com/aboard-io/stellate/commit/14daa065a3472c64e40311f6b5d3e79cad976c2b