Fix vm.sign malleable recovery value

[DicksonWu654]

Summary

• Toggle the malleable ECDSA recovery value between 27 and 28 with 55 - v.
• Strengthen the existing signature regression to require a valid, opposite recovery value.
• Keep the existing key, signature-range, and recovery constraints unchanged.

Rationale

The model constrains vm.sign recovery values to 27 or 28, but bitwise v ^ 1 maps them to 26 or 29. Those are not the alternate values in the 27/28 convention. Mapping 27↔28 correctly models the recovery-bit change paired with n - s.

Validation

• Static diff/whitespace validation passed.
• The regression expresses the expected toggle independently with a Solidity conditional and checks equal recovery.
• Executable tests, builds, package-manager commands, and hooks were not run locally; repository CI is expected to provide executable validation.