Mild-motion bounded recenter: seam survives sustained rides (Stage 15)

[robmorgan]

ROADMAP Stage 15's core item: the crossover seam no longer stays comb-filtered while riding the tempo fader.

Problem (K1, characterized first)

Engine-level probe across ride profiles: sustained mild rides (0.5–1% deviation — the mix-in gesture) let SOLA's drift sawtooth to the full 192-frame trigger, holding the 120 Hz seam at −7 dB for as long as the ride lasts. Rest recovery needs ~150 ms of stillness a moving fader never provides.

Ride profile	Worst seam loss	While riding
Plateau ladder 0.5–1%	−7.2 dB	recovers only at rest
Sine micro-ride ±0.6%	−7.1 dB	−2.5 dB, never recovers
Creep ±0.2% / nudge+park	−1 to −2 dB	fine (existing rest machinery)

Fix

When transposition deviation is mild (< 1.2%) and settled drift exceeds 96 frames, fire the bounded recenter splice (the rest-splice mechanism, without its dwell). The landing bound is load-bearing: a first attempt that simply tightened the general trigger made the seam worse (−5.5 dB persistent, even in the control profile) — unbounded early splices park drift on the dominant-period grid on periodic content, the same #62 pathology the bound exists to prevent. That negative result is documented in the constants.

Measured after

Profile	Before	After
Plateau ladder	−7.2 dB worst	−4.7 dB, full recovery
Sine micro-ride	−7.1 / −2.5 riding	−4.4 / −1.2
Creep, nudge+park	—	bit-identical

~5 bounded splices/s during mild rides (the shipped ±8% path already carries ~18/s). Cents-wobble and A/B-matrix gates pass unchanged.

Gates

• New seam_survives_a_sustained_mild_ride (keylock unit tests): worst > −6 dB, riding tail > −3 dB, splice-count bound. Fails pre-fix at −7.14 dB.
• qa/engine_keylock and qa/engine_transients promoted into the CI quality-gates job (Stage 15 exit criterion).

Remaining stage items (evidence-gated, separate PRs): correlation-reference fractional alignment, strength-aware protection, the modulation_hold wiring experiment, and the owner mix-in listen.

🤖 Generated with Claude Code