Restore safe SoNodeKitPath object semantics

[Dikluwe]

Stacked draft: this PR is based on the head of coin3d#714 and must be rebased/retargeted after coin3d#714 merges. The comparison intentionally contains only the two SoNodeKitPath commits.

Summary

• construct genuine SoNodeKitPath objects instead of treating SoPath storage as a derived object
• add SoNodeKitPath::fromPath() as the checked conversion boundary
• preserve the nodekit-facing path projection for length, node, index, tail, truncation, pop, append, equality, and I/O operations
• remove remaining manipulator code that treated an arbitrary SoPath* as SoNodeKitPath*

Motivation

SoNodeKitPath was used as an interface lens over storage created as SoPath. Unlike a valid base-to-derived relationship, the dynamic object was never a SoNodeKitPath; calling derived members through that pointer is undefined behavior and UBSan/vptr detects it. PR coin3d#714 fixes the analogous SoFullPath pattern and supplies the full-route accessors used by this follow-up.

This PR preserves the Open Inventor-facing projection while restoring a real C++ object model: objects returned as SoNodeKitPath* are now actually constructed as SoNodeKitPath.

Validation

• 11/11 focused contract tests pass in Debug and Release with UBSan/vptr
• 23/23 targeted mutation checks are killed by the contract suite
• the full CoinTests suite passes in Debug and Release
• ABI review shows no removals or layout changes; the public surface is additive (fromPath and operator!=)

Compatibility

The public nodekit projection remains intact. This is stacked on coin3d#714 because it depends on the new full-path accessors and must not be reviewed as a duplicate of that PR.

[Dikluwe]

Superseded by coin3d#747. The branch was rebased directly onto upstream master and the SoNodeKitPath fix is now independent from coin3d#714, so this stacked fork-only draft is no longer needed.