Refactor settings.rs to expose testable internal functions (get_settings_at,
save_settings_at) and add integration tests for file I/O roundtrips, whitespace
trimming, directory creation, and malformed JSON handling. Coverage for
settings.rs improved from 46% to 91%. Overall Rust coverage now 86.2% (above
85% threshold). Also add coverage/ to .gitignore.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
d2902ec6b4