feat(cheatcodes): Add vm.signWithNonce(privateKey, digest, nonce) cheatcode (Crypto)

[Ectario]

Motivation

Security researchers and advanced test writers sometimes need to create deterministic ECDSA signatures with a chosen ephemeral nonce k.

This is useful to:

• Reproduce signatures deterministically for debugging or reproducibility.
• Stress test and fuzz systems that rely on signature uniqueness.

Currently, Foundry's vm.sign only derives k internally (RFC6979) and does not allow injecting a fixed nonce.
For security testing and deeper protocol analysis, we need a way to supply k manually.

Closes #11266.

Solution

This PR introduces a new cheatcode:

function signWithNonce(uint256 privateKey, bytes32 digest, uint256 nonce)
    external
    returns (uint8 v, bytes32 r, bytes32 s);

• Signs digest on secp256k1 using privateKey and a user-supplied nonce k.
• Requires both privateKey and nonce to be in (0, n) where n is the secp256k1 curve order.
• Performs low-s normalization and recovery id computation.
• Intended primarily for security testing, fuzzing, and reproducing edge cases — not for production signing.

Changes include:

• crypto.rs implementation of the fixed-nonce signing path.
• Integration in the cheatcodes Vm.sol interface.
• New Solidity tests in testdata/default/cheats/Sign.t.sol covering valid and invalid nonce cases.

PR Checklist

• Added Tests
• Added Documentation

[Ectario]

Hi team,
Just wondering, was it intentional to not have cargo cheats included in the make pr target in the Makefile? (And is it expected that the first cargo cheats crashes but the second runs fine?)