X72-63: Fast-Difference Schur Elimination and Symmetrized Slow Gauge

Continuing from Round 62's idea of compressing the small-viscosity gap into a "fast-Schur graph + slow-Riccati" form (STOP-C66), this round switches to first-order parity-difference coordinates, no longer relying on the ill-conditioned local fast eigenbasis, and rewrites the third-order recurrence exactly in "slow forcing + contracting fast feedback" form. Using the coefficient frame already certified in Round 59, it directly proves the fast-feedback operator norms δ₋<0.005 and δ₊<0.075, so (I-K_fast) can be rigorously inverted by a Neumann series, and the fast sector is thereby eliminated exactly at the operator level, independent of viscosity (the "Fast-Difference Schur Theorem"), and finds that most of the asymmetry in the two-parity coupling can be removed by the gauge shift j↦j+3/4, compressing the remaining mismatch to O(j⁻²). The gap converges from three-dimensional bundle matching down to a selection problem for a single slow projection/Jost scattering line (STOP-C67), handed to the next round; this round includes numerical verification scripts and CSV data.