Fix cleanup of cross-object reactive watchers

[happykawayigt]

Link to issue or discussion

Fixes #6666

Summary

• track cross-object watcher registrations on the node that owns them
• unregister those watchers as soon as either the owner or observed node closes
• add regression coverage for both removal directions

Testing

• python -m pytest tests/test_reactive.py tests/test_data_bind.py tests/test_widget_removing.py tests/test_unmount.py tests/test_widget.py -q (98 passed)
• python -m black --check src/textual/reactive.py tests/test_reactive.py
• git diff --check