fix: ignore unaffected ruint advisory

[mattsse]

Summary

• add a documented exception for RUSTSEC-2026-0220
• preserve the crate's Rust 1.85 MSRV while restoring the cargo deny gate

Rationale

The advisory affects ruint's Uint shift operations and related no-alloc formatting behavior. alloy-chains only consumes alloy-primitives::Address, so it does not use the affected API surface.

Cargo resolves ruint 1.17.2 for this crate because the fixed ruint 1.20.0 release requires Rust 1.90. Raising the MSRV would be disproportionate for an unaffected transitive dependency; the narrowly scoped exception keeps that constraint explicit until the dependency can be upgraded.