chore: ignore macOS .DS_Store files

We've had a stray tests/.DS_Store showing up as untracked in
`git status` for several commits. The repo already has .gitignore
sections for Python / venv / IDEs / data / test artifacts but
nothing for the macOS file Finder drops into every directory it
touches. Add a minimal macOS section with just .DS_Store (skip
the broader `._*` resource-fork glob since the user asked for the
specific file).

No code or test changes — pure ignore-list update. The stray
.DS_Store on this machine has already been removed from disk, so
this commit touches only .gitignore.

Tests: 405 passed, no test changes (ignore list isn't covered by
tests).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -20,3 +20,6 @@ data/
 .pytest_cache/
 .coverage
 htmlcov/
+
+# macOS
+.DS_Store