C# properties got no graph node, so a property was invisible as a class member (only
fields and methods appeared). Emit a member node per property_declaration with the same id
scheme, ownership, and defines/field-context edge as the C++ data-member branch; all
property shapes (auto, expression-bodied, read-only) are covered, the #2913 property-type
references walk is preserved, and no builtin types are fabricated.
