fix(fusion): correct logit-scale comment; top_disease=None on empty input

feat(fusion): add core multi-modal fuse() with per-disease attribution

feat(fusion): map modality predictions to per-disease signals

feat(fusion): add clinical-test signal normalisers (MMSE/MoCA/UPDRS/gait/age)

feat(fusion): add disease/modality weight registry

fix(fusion): drop unused protected_namespaces; pin tighter bound tests

feat(fusion): add pydantic data contract for multi-modal fusion