Harden Windows signing against hostile fixed-base drift

[shrec]

What

• Routed nonce generator multiplication for ecdsa_sign, ecdsa_sign_hedged, and ecdsa_sign_recoverable through ct::generator_mul on MSVC.
• This isolates Windows signing correctness from hostile runtime fixed-base reconfiguration.
• The change closes the remaining windows (Release) CI blocker triggered by exploit_ctx_lifecycle_hostile.

How to get it

• Branch: main
• Commit: 65075143