Modernize dependencies and Unicode support

[hanwencheng]

What changed

• update Criterion from 0.3 to 0.8.2 and move the crate to Rust 2024 with MSRV 1.86
• make substring construction and length calculations Unicode-safe
• add threshold validation and stronger regression tests
• resolve current Clippy and rustfmt findings
• add CI for formatting, Clippy, tests, doctests, and benchmark compilation
• add weekly Dependabot updates

Validation

• cargo fmt --all -- --check
• cargo test --all-features (5 integration tests and 1 doctest)
• cargo clippy --all-targets --all-features -- -D warnings
• cargo bench --no-run
• cargo package --allow-dirty