fix: bump CI Python from 3.11 to 3.12

pyproject.toml requires-python was raised to >=3.12 and PEP 695 type aliases (type X = Y) were introduced in domain.py, midi_merge.py, and midi_parser.py — all of which are SyntaxErrors on Python 3.11. CI was still pinned to 3.11, causing every job to fail at collection.