composition_dispersion is not a reserved uns name, and hit_calling's Raises is out of date #91

Three small gaps left by #88.

tl.cluster_composition writes uns["mantispy"]["composition_dispersion"] and documents it, but the name is not reserved:

• src/mantispy/_core/schema.py:129 lists composition_test in UNS_RESULTS and not composition_dispersion
• spec/schema-1.0.json:101 mirrors the same omission
• docs/api.md:179 lists only uns["mantispy"]["composition_test"] for this function

Nothing breaks, since the list is not enforced during validation, but every other documented output is reserved.

tl.hit_calling's Raises: section does not mention the two errors the covariance parameter can raise: the ValueError for a value outside COVARIANCES, and the ValueError from mahalanobis_transform when covariance="robust" gets no more complete reference rows than features.

src/mantispy/tl/_heterogeneity.py names _DISPERSION_MIN_CONTROLS in the public docstring of cluster_composition, so the rendered page points at a private module constant the reader cannot import. The number belongs there instead.