RestingLayoutCache (session-scoped, @ObservationIgnored on DragSession,
cleared at begin and end) holds each lane's resting layout as ids +
heights — never [Card] — keyed on the hovered board's applied
snapshotGeneration, the registry's new heightsGeneration, the lane, and
the operation-following hidden set. The grid stays event-time on purpose
(autoscroll moves a lane's origin with the snapshot standing still), so
re-grounding rule 1 holds exactly: nothing survives a reload, only the
per-sample repetition goes. The steady-state cost of a hover is now the
containment scan plus four stores and a divide.
Drag-perf suspect #3, card b9f48fd1.
Claude-Session: https://claude.ai/code/session_01CqjXB7ASoWtbyoGod68k97
