Proposal changes stop rebuilding the whole board — lanes and card faces gate on their values

LaneView and CardFaceView become Equatable and are instantiated through
.equatable(): the strip's body re-runs on every drop-proposal change, and
without the gates that rebuilt every lane and every card face on every
cursor move of a drag. The == compares value inputs and window-lived
collaborator identities; the closures BoardView rebuilds each pass are
deliberately excluded (BoardDropContext.isEquivalent / MarqueeControl.
isEquivalent / CardFaceRole.isEquivalent own that judgment). Observation
reads inside the bodies still self-invalidate — the lane under the drag
keeps re-running; the other lanes stop.

Ports the pathfinder's CardView/ColumnView gating pattern (drag-perf
suspect #2, card cbb6e476).

Claude-Session: https://claude.ai/code/session_01CqjXB7ASoWtbyoGod68k97

Kanban/UI/Board/SelectionClicks.swift

@@ -40,6 +40,17 @@ struct MarqueeControl {
     let registry: MarqueeTargetRegistry
     let store: BoardStore
 
+    /// Whether two of these lend the same band — the whole of what this value contributes to
+    /// `LaneView.==` and `CardFaceView.==` (`BoardDropContext.isEquivalent(to:)` is its twin).
+    ///
+    /// All three members are window-lived objects, so identity is the comparison: this struct holds
+    /// no geometry and no closures of its own, and the strip rebuilds it on every body pass.
+    nonisolated func isEquivalent(to other: MarqueeControl) -> Bool {
+        session === other.session
+            && registry === other.registry
+            && store === other.store
+    }
+
     /// The band, as one gesture attached with `simultaneousGesture` wherever empty space is.
     ///
     /// - **The begin guard is geometric**: a drag whose start lands inside a registered frame is