Fix SoPath assignment auditor lifecycle

[Dikluwe]

Summary

• keep the destination path and its existing child-list auditors intact while reserving capacity and registering the new route
• roll back partially registered auditors if an allocation fails
• remove the old auditors before replacing the route, while preserving self-assignment and copy construction
• merge the current master after Validate SoPath auditor callbacks in all builds #749 and retain both test-registration blocks

Tests

• regression cases cover reassignment, self-assignment, copy construction, allocation failures during list growth and auditor registration, shared roots, shorter and empty paths, and transitions from SoTempPath
• the combined Fix SoPath assignment auditor lifecycle #748 + Validate SoPath auditor callbacks in all builds #749 Release suite passes: 31/31 CTests
• the nine focused assignment and callback tests pass with AddressSanitizer and UndefinedBehaviorSanitizer
• ABI compatibility, CodeQL, Ubuntu, Windows, and macOS checks pass on the current PR head

Compatibility

No public API or object-layout change.

[veelo]

Thanks.