Restore safe SoNodeKitPath object semantics

[Dikluwe]

Summary

• construct genuine SoNodeKitPath objects instead of exposing SoPath storage through a derived pointer
• add SoNodeKitPath::fromPath() as an explicit copying boundary
• preserve nodekit projection, mutation, comparison, fork, and I/O behavior
• remove unsafe manipulator casts

Tests

• registered SoNodeKitPathBehavior CTest covers materialization, independent ownership, projected accessors, append through hidden nodes, and factory dynamic type
• strict-warning Debug build and the focused CTest pass

Compatibility

The inheritance and object layout are preserved. The API additions are fromPath() and operator!=.