The resting layout is built once per snapshot — retargets stop rebuilding it per mouse sample

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

Kanban/UI/Board/DragSession.swift

@@ -323,6 +323,20 @@ final class DragSession {
     /// whichever board's standard width it is being proposed into.
     @ObservationIgnored private(set) var laneUnits: [Int] = []
 
+    /// The per-lane resting layouts this drag's retargets propose against, built once per snapshot
+    /// rather than once per mouse sample (`RestingLayoutCache`, which states why that leaves rule 1
+    /// of the re-grounding trio exactly as it was).
+    ///
+    /// `@ObservationIgnored` for the reason `LaneDropRegistry` is not `@Observable` at all: it is
+    /// written from *event* handlers, and a cache fill that invalidated the strip would be the
+    /// animation feedback loop this whole model exists to avoid (03-board-ui.md § Motion).
+    ///
+    /// **Session-scoped**, so it cannot outlive the drag it was filled for — and so a layout can
+    /// assume the one input that is neither in its key nor read live, `cardHeights`, is the frozen
+    /// set this session picked up with. `begin` and `end` bracket every session this object has, and
+    /// both clear it.
+    @ObservationIgnored let restingLayouts = RestingLayoutCache()
+
     // MARK: Where it would land
 
     /// The current proposal, or `nil` when the drag has none — a fresh session before the first
@@ -564,6 +578,7 @@ final class DragSession {
         mixesKinds: Bool
     ) {
         endHold()
+        restingLayouts.clear()
         self.kind = kind
         self.members = members
         self.memberSet = Set(members)
@@ -627,6 +642,7 @@ final class DragSession {
         folders = []
         cardHeights = []
         laneUnits = []
+        restingLayouts.clear()
         proposal = nil
         operation = .move
         sourceStore = nil