X72-62: Neutral Residual Cancellation and the Quarter-Power Matching Law

Continuing from Round 61's approach of compressing the error budget into a fast-Schur graph and a slow-Riccati part (STOP-C65), this round examines the remainder that actually acts along the neutral direction after Schur elimination, proving that the neutral residual cancels exactly down to S_n=48K³/n³+O(n⁻⁴) (rather than the originally estimated O(n⁻²)), so the neutral-root drift is only cubic order, and derives the asymptotic viscous-coupling corrections 1+j⁻¹ and 1+2j⁻¹ for the two parities. On this basis it rebalances the three post-Schur error terms, correcting the optimal overlap exponent from Round 61's 2/7 back to α*=1/4, restoring the "quarter-power matching law" as a structurally self-consistent theorem target; the six-dimensional principal-angle numerics remain far better than this conservative bound even at ν=10⁻⁸. This round still does not prove the full O(ν^(1/4)) theorem; the gap shifts to STOP-C66, left for the next round to construct an explicit invariant fast graph; this round includes numerical verification scripts and CSV data.