refactor: resolve remaining strict Clippy findings

[shayyz-code]

Summary

• resolve the remaining strict Clippy diagnostics in the lexer and parser
• clean type-inference diagnostics and add Default for the scoped symbol table
• record the clean local strict-Clippy baseline

Issue

Closes #9

Verification

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

Documentation and visual impact

Roadmap status only; no visual impact.

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