#947 PDM: Add caf::PdmNestedCollection template and migrate surface + polygon collections

[magnesj]

Summary

Introduces a CRTP template caf::PdmNestedCollection<SelfT, ItemT> that extends caf::PdmObjectCollection<ItemT> with a user-visible collection name and a vector of subcollections of the derived type. This generalizes the "name + items + subcollections-of-self" pattern previously hand-rolled in RimSurfaceCollection and RimPolygonCollection.

A non-templated caf::PdmNestedCollectionInterface lets generic command features operate on any nested collection without knowing the template parameters. Includes a canAddSubCollection() query so leaf containers (e.g. file-backed folders) can refuse new sub-collections and let the generic UI hide the corresponding action.

A parallel CRTP template RimNestedMirrorCollectionInView<SelfT, SourceT, ItemViewT> collapses the hand-rolled in-view mirror sync logic in RimPolygonInViewCollection into a few small overrides. The same template can drop into RimSurfaceInViewCollection next.

XML keywords are kept in the derived constructors so existing project files load unchanged. Legacy RimPolygonCollection::m_polygonFiles is migrated into the new m_subCollections in initAfterRead.

Changes

caf framework

• New Fwk/AppFwk/cafProjectDataModel/cafPdmNestedCollection.{h,inl} — CRTP template providing collectionName/setCollectionName, subCollections, addSubCollection, addNewSubCollection, findSubCollectionByName, allItems, setAsTopmostFolder, plus userDescriptionField and defineUiOrdering overrides.
• New Fwk/AppFwk/cafProjectDataModel/cafPdmNestedCollectionInterface.h — non-templated interface for generic features (canAddSubCollection, addNewSubCollection, collectionName/setCollectionName).
• Modified cafPdmObjectCollection.h/.inl — replace is_base_of static_assert with a C++20 DerivedFromPdmObject concept, evaluated lazily inside the constructor so derived types may be forward-declared at the point the class template is instantiated as a base.

Generic command

• New RicNewNestedCollectionFeature.{h,cpp} — generic "Add Folder" command that operates on any PdmNestedCollectionInterface and is disabled on leaves via canAddSubCollection().
• Removed RicNewSurfaceCollectionFeature.{h,cpp} — superseded.

Surfaces

• Modified RimSurfaceCollection.{h,cpp} — inherit from PdmNestedCollection, drop ~80 lines of duplicated boilerplate; preserves setAsTopmostFolder semantics.
• Modified RimEnsembleSurface.cpp — getSubCollection → findSubCollectionByName.

Polygons

• New RimPolygonContainer.{h,cpp} — XML-abstract base for both RimPolygonCollection (folder) and RimPolygonFile (file-backed leaf folder). Both live polymorphically in the inherited m_subCollections, so the in-view mirror picks them up uniformly.
• Modified RimPolygonCollection.{h,cpp} — inherit from RimPolygonContainer; legacy m_polygonFiles field migrated into m_subCollections in initAfterRead and disabled for write. userDefinedPolygons() removed in favor of allPolygons().
• Modified RimPolygonFile.{h,cpp} — inherit from RimPolygonContainer; overrides canAddSubCollection() → false. Reuses "Name" keyword for m_collectionName for backward compatibility.

In-view mirror

• New RimNestedMirrorCollectionInView.{h,inl} — CRTP template for view-side mirrors of nested source collections. Derived classes adapt to their source type via three small hooks.
• Modified RimPolygonInViewCollection.{h,cpp} — migrated to the new template; ~150 lines of hand-rolled sync removed.

Docs

• New docs/pdm-nested-collection-design.md — records the line-count comparison and the trade-off behind keeping the templated design over a non-templated alternative.

Closes #947.