X72-60: Singular Boundary-Layer WKB Matching Law

Continuing from the endpoint Green-functional positivity proved in Round 59, this round addresses the gap it left behind — the singular matching limit a₃(ν)/ν→c₀ at fixed ν. It finds that the true neutral-to-minimal decay layer is not the ν^(-1/2) scale suggested by the original coarse Banach estimate, but rather the cubic WKB decay scale ν^(-1/3), derives the reduced 2×2 slow-pair transfer matrix and its exact stable multiplier λ₋(a)=e^(-2 arsinh(a/2)), and verifies this scaling's convergence with direct numerical solution of the full recurrence (at ν=10⁻⁷ the observed/predicted half-decay ratio reaches 0.987–0.995). This round explicitly states that the matching limit has not yet been elevated to a theorem; the gap shifts to STOP-C64 (the WKB stable-bundle/rigorous-matching gap), left for the next round; this round includes numerical verification scripts and CSV data.