chore: Fix lint errors in lookup.rs

[anakrish]

• Made the lookup module crate-visible to address clippy’s redundant visibility lint.
• Replaced unchecked as casts with a fallible usize_from_u32 helper and propagate conversion errors in lookup accessors.
• Switched LookupIndexError to implement core::error::Error for no_std correctness.
• Fixed the pattern type mismatch by matching on the value in the Display impl.
• Promoted trivial helpers to const fn (new, module_len) per clippy suggestions.
• Centralized bounds-checked slot access via slot_ref/slot_mut to keep getters/clearers lint-clean and avoid unchecked indexing.

[Copilot]

Pull request overview

This PR addresses clippy lint errors in the lookup module by improving type safety, error handling, and code quality without changing the module's external behavior.

Key changes:

• Replaced unsafe as casts with checked try_from conversions and proper error propagation
• Changed from std::error::Error to core::error::Error for no_std compatibility
• Improved API encapsulation by making the lookup module and its methods crate-visible

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/lookup.rs	Removed clippy allow directives; added fallible usize conversion helper; changed visibility to pub(crate); promoted trivial functions to const fn; improved bounds checking with new slot_ref/slot_mut helpers; fixed Display impl pattern matching
src/lib.rs	Changed lookup module visibility from private to pub(crate) to match the module's intended scope

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