fix: add rhythmic as 5th music domain dimension everywhere

The MIDI merge has always recognised five user-facing dimensions (melodic, rhythmic, harmonic, dynamic, structural) via DIM_MAP in midi_merge.py, but schema(), its docstring, the Domain Registry card, and the test fixture all listed only four.

- plugin.py schema(): add DimensionSpec(rhythmic) with SequenceSchema matching melodic; note the shared internal notes bucket in description - plugin.py docstrings: update "four" → "five" in schema() and merge() - render_domain_registry.py: add rhythmic row to the music domain card - test_domain_schema.py: rename test_schema_has_four_dimensions → test_schema_has_five_dimensions; extend expected names list