fix: naming-convention false positive on ERC-7201 $ storage pointer

[innbuld]

Summary

The naming-convention detector reports the identifier $ as is not in mixedCase. $ is a valid Solidity identifier and the de-facto convention for the ERC-7201 namespaced-storage struct pointer (OpenZeppelin upgradeable, Solady, and most modern contracts). It has no letters to case, so it should be treated as conformant, consistent with the detector's other pragmatic exceptions (ERC20 lowercase names, leading _, i_/s_).

Impact

Any project using ERC-7201 namespaced storage gets one false positive per function f(SomeStorage storage $). Solady alone emits 24+ Parameter ... .$ is not in mixedCase findings, drowning out real results.

Minimal repro:

library LibExample {
    struct BytesStorage { bytes32 _spacer; }
    function length(BytesStorage storage $) internal view returns (uint256 r) {
        assembly { r := shr(224, sload($.slot)) }
    }
}

$ slither LibExample.sol --detect naming-convention
Parameter LibExample.length(LibExample.BytesStorage).$ is not in mixedCase

Fix

Exempt the exact identifier $ in is_mixed_case and is_mixed_case_with_underscore. The exemption is intentionally limited to the lone $ (the demonstrated convention) and does not broaden to arbitrary $-containing names, so no genuine violations are silenced.

Testing

• Adds naming_convention_dollar_storage.sol, covering $ as a read parameter, $ as an unused parameter, and a snake_cased tripwire parameter that must still be flagged. The snapshot contains exactly the tripwire finding.
• Full detector e2e suite passes (385 tests); ruff check/ruff format clean.

[CLAassistant]

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