feat: fix some bugs, improve some perf, add some tests

[ParthJadhav]

• Add 32 integration and unit tests covering SearchBuilder, filters, FileSize conversions, and utility functions — the crate
  previously had zero tests
  • Fix 6 bugs: panics in file_name_from_path and similarity_sort, broken per-thread limit counter, non-idiomatic Into impl,
    unnecessary allocations
  • Modernize CI workflow (replace deprecated actions-rs with direct rustup, upgrade to actions/checkout@v4, run lint in
    parallel)