perf(utils): build Yul storage mapping pointer generator for nonces

[SharifIbrahimDev]

Summary

This PR implements a gas-optimized Yul storage mapping pointer generator for message nonces (mapping(uint256 chainId => mapping(uint256 nonce => bool))), resolving issue #849.

Closes #849

Changes

1. contracts/utils/NonceSlotCalculator.sol [NEW]

Implements a Solidity library NonceSlotCalculator with functions:

• slotForNonce(uint256 chainId, uint256 nonce): calculates storage slot assuming default mapping slot 0.
• slotForNonceAt(uint256 chainId, uint256 nonce, uint256 mappingSlot): calculates storage slot for any arbitrary mapping storage slot.

Key Optimizations & Characteristics:

• Zero Memory Allocation: Uses EVM scratch space (0x00–0x3f) to stage keys and intermediate hashes.
• In-place Hashing: Computes outerSlot = keccak256(chainId ‖ mappingSlot) at 0x00–0x40, and subsequently innerSlot = keccak256(nonce ‖ outerSlot) by reusing the same scratch space slots.
• Preserves Free Memory Pointer: Does not touch or advance 0x40, eliminating memory expansion gas costs for mapping lookups.
• Solidity Storage Layout Compliant: Conforms to standard EVM nested mapping storage slot derivations.

2. test/utils/NonceSlotCalculatorHarness.sol [NEW]

Test harness contract exposing NonceSlotCalculator functions and verifying calculated slots directly against on-chain storage via sload.

3. test/utils/NonceSlotCalculator.test.ts [NEW]

Comprehensive unit tests using Hardhat, Mocha, and Chai:

• Validates calculated storage slot matches actual slot written by Solidity for nonces[chainId][nonce].
• Checks boundary and edge cases: (0, 0), large uint128 values, and type(uint256).max.
• Validates slot uniqueness across distinct key pairs.
• Validates arbitrary mappingSlot offsets with slotForNonceAt.
• Verifies function state mutability (pure) guaranteeing zero memory expansion.

Test Results

  NonceSlotCalculator
    ✔ computes the slot that Solidity actually writes to for nonces[1][100]
    ✔ matches the reference slot for chainId=0, nonce=0 (zero keys)
    ✔ matches the reference slot for large chainId and nonce values
    ✔ matches the reference slot for max uint256 keys
    ✔ produces unique slots for distinct (chainId, nonce) pairs
    ✔ slots differ when only chainId changes
    ✔ slots differ when only nonce changes
    ✔ slotForNonceAt matches slotForNonce when mappingSlot is 0
    ✔ slotForNonceAt produces different slots for different mappingSlot values
    ✔ slotForNonceAt matches off-chain reference for non-zero mappingSlot
    ✔ does not advance the free memory pointer (0x40) during slot computation

  11 passing (9s)

[mijinummi]

LGTM!