Fix Unix-socket test failing on macOS due to AF_UNIX path length (#14)

tmp_path nests too deeply for AF_UNIX socket paths (~104-108 byte
limit on macOS). Use a flat tempfile.TemporaryDirectory(), matching
the pattern already used by TestEntryKind::test_socket.

Co-authored-by: Maxim Shulga <maxim.shulga@alludo.com>
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

IMPROVEMENTS.md

@@ -157,7 +157,7 @@ Acceptance criteria:
 
 - [ ] Run CI on pull requests as well as pushes.
 - [ ] Add macOS CI because macOS is an advertised platform.
-- [ ] Fix the Unix-socket path test that currently fails on a local macOS run.
+- [x] Fix the Unix-socket path test that currently fails on a local macOS run.
 - [ ] Replace nonexistent issue references in `xfail` markers with real tracked
   work, then remove the markers as fixes land.
 - [ ] Add subprocess-based concurrency, signal, interruption, and fault tests.