perf: precompute Jaq dot-path keys

[daisuke8000]

Summary

• Precompute Jaq dot-path keys once when configuring level classification.
• Reuse the keys for each log line while preserving Serde pointer lookup behavior.
• Cover nested paths and empty-segment compatibility with focused tests.

Verification

• cargo fmt --all --check
• cargo audit
• cargo build --workspace --locked
• cargo test -p rustern-core --lib
• cargo clippy --all-targets -- -D warnings