Table-anchored nested resource for a Planning Table's datasets.

A dataset is never addressed by its own id here: the route is keyed on the parent table (table_pk) and a dataset is identified by (table, scenario_id from the request). Hence get_queryset/get_object return the parent PlanningTable (access control -- sharing scheme and created_by -- lives on the table), while the actions query PlanningTableDataset rows directly.

Consequence for permission_classes: model-derived primitives (ModelPermissions, UserHasModelShared) resolve model_name from the queryset, so they check *_planningtable perms, NOT *_planningtabledataset. Only the hardcoded SpecificModelPermissions / EditSharedPermissions target the dataset model. That is why the expression below mixes table-level and dataset-level permissions.