chore: complete final protocol hardening pass

[alsaecas]

Summary

Performs a repository-wide final hardening pass while preserving the protocol architecture and settlement implementation.

• strengthens adversarial EIP-712, ERC-1271, token-failure, rounding, and lifecycle tests;
• makes the invariant handler attempt forbidden fills after cancellation, nonce invalidation, and completion;
• pins current OpenZeppelin, Slither, and GitHub Action versions;
• aligns local and CI static-analysis policy and fixes CI-profile compatibility with older Foundry releases;
• expands security, contribution, testing, and static-analysis documentation.

Security considerations

The settlement contract itself is unchanged: the audit did not identify a correctness flaw that justified modifying production logic. New regressions prove malformed or mutated signatures fail, chain-domain replay fails, false-returning token operations roll back atomically, arbitrary two-part fills settle the exact signed buy amount, and all terminal states block later fills.

The documented Slither exclusions remain limited to arbitrary-send-erc20 and timestamp, with the authorization and expiry rationale recorded in docs/STATIC-ANALYSIS.md.

Validation

Executed locally with Solidity 0.8.30 and the CI Foundry profile:

• forge fmt --check — pass
• forge lint — pass (informational ABI-hashing and test naming notes reviewed)
• forge build --sizes — pass
• forge test -vvv — 44 passed: 35 unit, 4 fuzz, 5 invariant
• fuzz configuration — 4,096 runs per property
• invariant configuration — 512 runs, depth 128 (65,536 calls per invariant)
• production src/ coverage — 100% lines/statements/branches/functions
• repository aggregate — 94.23% lines, 93.96% statements, 86.21% branches, 94.87% functions
• Slither 0.11.6 — 27 contracts, 100 active detectors, 0 results after the two documented exclusions

Release note

This remains educational/reference software, has not undergone a professional security audit, and should not be used with meaningful funds.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep make check usable with the documented prerequisites

On a fresh clone following the README quick start, the user installs only Foundry dependencies and then runs make check, but this new dependency invokes slither, which make install does not install and the README does not list as a requirement. The advertised quick-start command therefore ends with slither: command not found; either keep static analysis in the explicit slither/ci targets or update the quick-start prerequisites and setup to install Slither.

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