FrameCutoffNode

Retained capability that publishes external observations at the start of every timed or untimed session frame.

Callbacks run on the tree owner thread before reconciliation, measurement, or frame-cache lookup and must not mutate session state. Every participating entry is captured before any entry is committed, so publication triggered by a commit cannot enter a later entry in that frame. A callback failure poisons the retained tree and preserves the original exception through best-effort cleanup.