Rename package directory AbletonOSC/ -> ableton_osc/

Python package naming convention uses snake_case. Update the import in
the root __init__.py and the setuptools include pattern in pyproject.toml.
Internal relative imports within the package are unaffected.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

__init__.py

@@ -1,5 +1,5 @@
 try:
-    from .AbletonOSC.manager import Manager
+    from .ableton_osc.manager import Manager
 except ImportError:
     # Allows pytest to import without Ableton's runtime present
     pass