X72-65: Banded A Posteriori Validated Viscosity Extension

Continuing from the half-line theorem a₃,±(ν)>0 (ν≥10⁻⁴) proved in Round 64, this round replaces dense interval matrix inversion with banded a posteriori certificates and incorporates an IEEE-754 rounding-error model, lowering the verification cost from O(N²), thereby closing two further decades ([10⁻⁵,10⁻⁴] and [10⁻⁶,10⁻⁵]). This gives a₃,±(ν)>0 for all ν≥10⁻⁶; combined with the same-sign Fredholm pairing from Round 55, the full second-order analytic hidden remedy of the two √17 hidden source circles is ruled out within this range. The remaining singular band narrows to 0<ν<10⁻⁶, and the bottleneck shifts from residual verification to the storage cost of the dense approximate inverse matrix, left for the next round to handle with fixed-size block-Riccati certificates; this round includes numerical verification scripts and CSV data.