Add Polymorphic Child List documentation

[rockfordlhotka]

Summary

Adds a new documentation page, csla-examples/PolymorphicChildList.md, describing how to build a BusinessListBase root list that safely holds polymorphic (heterogeneous) editable child types — e.g. a SurveyResultList containing both AssetSurveyResult and ContactSurveyResult, which share a SurveyResult base.

The document is placed in the common (version-neutral) area and linked from the README's Topic-Specific Documents table.

What it covers

The pattern works, but only if three CSLA-specific constraints are met. Each is documented along with the exact runtime/compile error produced if violated:

1. CRTP base required — the shared base must be SurveyResult<T> where T : SurveyResult<T>. A non-generic shared base throws Cannot register property X after containing type has been instantiated once a second subclass adds managed properties.
2. List item type must extend IBusinessBase — typing the list against an interface that extends only IEditableBusinessObject compiles but throws Cannot serialize collections not of type IMobileObject in MobileFormatter.
3. Properties must be hand-coded — [CslaImplementProperties] emits an empty partial for any type whose direct base is a user-defined class, so managed properties must use RegisterProperty/GetProperty/SetProperty.

Validation

Every claim was verified against a working CSLA 10.1.0 sample project (fetch of a mixed list, Clone()/MobileFormatter round-trip preserving concrete types, and save via Child_Update()).