ci: enforce strict Clippy checks

[shayyz-code]

Summary

• enforce strict Clippy in the existing protected Cargo check job
• preserve the formatting, compilation, and complete test baselines
• mark strict-Clippy CI enforcement complete in the roadmap

Issue

Closes #11

Verification

• cargo fmt --all -- --check
• cargo check --all-targets
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test
• actionlint

Documentation and visual impact

Roadmap status only; no visual impact.

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