Post Quantum Migration and Legacy Signature Sunset.

Would phase out today's signatures once quantum-safe addresses exist: first blocking payments to vulnerable addresses, then requiring a rescue proof.

A quantum computer could steal coins whose public keys are already visible. Phase A, about three years after activation, would stop payments to quantum-vulnerable addresses. Phase B, two years later, would let old-style signatures spend only with an extra quantum-safe 'rescue' proof. Coins never moved by owners who can't give one would stay locked.