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/BoardDrops.swift

@@ -110,6 +110,23 @@ struct BoardDropContext {
     /// The strip's 1× lane width for the current drag context (`LaneLayoutMath.standardWidth`).
     let standard: @MainActor () -> CGFloat
 
+    /// Whether two of these would send a drop to the same place — **the whole of what this value
+    /// contributes to `LaneView.==` and `CardFaceView.==`.**
+    ///
+    /// `BoardView.dropContext` rebuilds this struct on every body pass, closures and all, so a naive
+    /// value comparison is impossible and a reference comparison is meaningless: the three closures
+    /// are freshly allocated each time and would make every lane and every card face unequal on every
+    /// pass, which is exactly the rebuild the gates exist to stop. They are also the members it is
+    /// safe to ignore — each one *reads* window and strip geometry at event time rather than carrying
+    /// any (see this type's own note), so two contexts with the same store, session and registry
+    /// resolve every hover identically whatever closure objects they happen to hold.
+    nonisolated func isEquivalent(to other: BoardDropContext) -> Bool {
+        store === other.store
+            && session === other.session
+            && registry === other.registry
+            && gap == other.gap
+    }
+
     // MARK: The cursor
 
     /// The physical cursor in the window's SwiftUI global space.