Add UI core node lifetime handles

docs/modernization/roadmap.md

@@ -467,7 +467,11 @@ and lookup pointers, public mutable child ownership, raw callback targets, and
 manual destroy flags. `pp_ui_core` should introduce owned tree/handle APIs,
 scoped callback connections, mutation-safe event dispatch, and focused
 destroy-during-callback tests before broad `NodePanel*`/`NodeDialog*`
-migration accelerates.
+migration accelerates. The first pure `pp_ui_core::NodeLifetimeTree` slice now
+models checked handles, parent/child invariants, scoped callback connections,
+and mutation-safe dispatch for destroy-during-callback and connection-addition
+cases; wiring those semantics into retained `Node` remains tracked by the same
+debt.
 `pano_cli inspect-image` exposes PNG IHDR metadata as JSON,
 `pano_cli import-image` accepts a PNG path and imports decoded RGBA8 pixels
 into a new pure `pp_document` face payload,