Introduce a shared base for tree-shaped folder containers that hold a
list of leaf items together with a list of subcontainers of the same
kind. Replaces the per-class plumbing (subcollections field, top-folder
flag, add/remove/find sub-collection, factory) that is duplicated across
RimSurfaceCollection, RimPolygonCollection and similar candidates.
- caf::PdmNestedCollectionBase: non-templated PdmObject base exposing
collection-name, add-new-subcollection, leaf-check and topmost-folder
marking for generic command features and script methods.
- caf::PdmNestedCollection<SelfT, ItemT>: CRTP template that owns the
typed m_subCollections field and inherits the m_items array from
PdmObjectCollection<ItemT>. Provides allItems(), find-sub-collection,
and a typed factory hook.
- RimNestedMirrorCollectionInView: view-side analogue for in-view
collections that mirror a nested source collection, with orphan-sweep
+ find-or-create + reorder sync.
Design rationale (templated vs non-templated, break-even on derived
class count) lives in docs/pdm-nested-collection-design.md.
No callers migrated in this commit.
