[QIU-Guanzong]

Recognizes .nx as an Nx cache only with a sibling nx.json; a package.json alone is insufficient. Adds .nx to the default traversal list because it is still absent on current main. The real-filesystem regression test covers default scans and two unmarked lookalikes.

Closes #4.

Validation: cargo fmt --all and cargo test passed (314 tests including doc tests). All-feature Clippy was attempted; the unchanged macOS unnecessary_cast errors in src/platform/unix.rs block it (tracked in #5, reproduced on the unmodified base). No cleanup code changes.

Prepared with AI assistance and locally verified on macOS arm64 / Rust 1.88.0.