Use persistent property in deferred IMGUI context-menu callbacks

[VPDPersonal]

Summary

• Fix three deferred GenericMenu callbacks in the IMGUI serialize-reference drawer that captured the stale per-frame property instead of the persistent snapshot the drawer already builds for the menu.
• Create New Script…, Paste Template/*, and Link to Existing/* now read/write through persistent, so by click time they no longer dereference a SerializedObject that may have been disposed after the frame.

Notes for review

• LinkTo reads its source via property.serializedObject.FindProperty(sourcePath) and writes back on the same SerializedObject; passing persistent keeps both sides on one live SO and preserves the shared managedReferenceId.
• ApplyTemplate still calls property.Persistent() internally — now harmlessly idempotent since it receives an already-persistent property, and the deferred callback dereferences a live SO.

Linked issues

Refs #49 - addresses review finding #49 (comment)