v0.2.1: Code review fixes, platform-neutral docs

- Fix f-string logging → %-style (face_protector, invisible_engine)
- Fix logger name: hardcoded string → __name__
- Add module docstrings to humanizer.py, face_protector.py
- Break long warning string into multiple lines (PEP 8)
- Make docs platform-neutral (macOS/Linux/Windows)
- Rename 'optional' → 'additional setup' in README

tests/test_cli.py

@@ -79,7 +79,7 @@ class TestMainGroup:
     def test_version(self, runner):
         result = runner.invoke(main, ["--version"])
         assert result.exit_code == 0
-        assert "0.2.0" in result.output
+        assert "0.2.1" in result.output
 
     def test_no_command_shows_banner(self, runner):
         result = runner.invoke(main, [])