Add a macos-latest job to the CI matrix

[darkone-linux]

Summary

All three CI jobs run on ubuntu-latest (.github/workflows/ci.yml), so every
platform-conditional branch in the codebase ships untested:

• rename_no_clobber (src/main.rs:392-401) — the #[cfg(not(target_os = "linux"))] fallback, try_exists() + std::fs::rename, has never been
  executed by CI. It is the branch implicated in Dangling symlink at the destination escapes conflict detection #5.
• is_same_entry (src/lib.rs:769) — the #[cfg(not(unix))] arm, likewise.
• Case-insensitive filesystem behaviour, which is what makes Case-only renames rejected on case-insensitive filesystems (macOS) #4 unreproducible
  and unfixable today: a correction there would be a blind change to the only
  code path allowed to touch the filesystem.

macos-latest closes all three at once: APFS is case-insensitive by default,
the runner is arm64 (some aarch64 coverage, which the flake claims to support
but never builds in CI), and it exercises the non-Linux rename fallback.

Suggested scope

A test-only job, not a second copy of the full gate. fmt, clippy,
audit and doc are platform-independent; re-running them buys nothing and
doubles the minutes. Nix is unnecessary too — dtolnay/rust-toolchain +
cargo test --locked is enough, and much faster than provisioning Nix on a
macOS runner.

  macos:
    name: Tests (macOS)
    runs-on: macos-latest
    steps:
      - uses: actions/checkout@v7
      - uses: dtolnay/rust-toolchain@stable
      - run: cargo test --locked

Expected fallout — read this before landing

Adding the job turns CI red immediately. The failures are real information, not
noise, but they need handling in the right order.

Fixtures that collapse on a case-insensitive filesystem. These tests create
two entries in one directory whose names fold together, so the second write
overwrites the first and the fixture silently becomes a single file:

Fixtures in the same directory	File
CAFÉ.TXT / Café.txt / café.txt	tests/cli_tests.rs
CAFE.txt / cafe.txt	tests/cli_tests.rs
Fichier.txt / fichier.txt	tests/cli_tests.rs
Visible.txt / visible.txt	tests/cli_tests.rs

These are test-harness bugs, fixable on their own: pick fixture names that stay
distinct under case folding (that is what most of the suite already does —
Fichier Test.txt → fichier-test.txt differs by more than case).

Assertions blocked on #4. Visible.txt → visible.txt, Fichier.txt →
fichier.txt and CAFE.txt → cafe.txt are case-only renames, which macOS
currently refuses (#4). Those assertions cannot pass until #4 is fixed.

Suggested order:

1. Fix the fixture collisions above (green on Linux, no behaviour change).
2. Land the macOS job with the case-only assertions marked
   #[cfg_attr(target_os = "macos", ignore)], referencing Case-only renames rejected on case-insensitive filesystems (macOS) #4.
3. Fix Case-only renames rejected on case-insensitive filesystems (macOS) #4, then drop the ignore — the test suite becomes the proof that the
   fix works, which is exactly what is missing today.

Blocks #4. Related: #5.