A refused trash hover withdraws the trash's own promise — the fall-through holds only keepable ones

Gap #12 ruled into 04 ▸ the trash-drop bullet: the refusal fall-through's
honest reading (the column declines to be a target, it does not cancel
the held drag) covers only promises the refusal leaves keepable — lane
shadows keep drawing and an ⌥-copy lands where they are. A standing
proposal naming the trash itself is the promise the refusal just broke,
so it withdraws: the drag goes proposal-less over the column, release
with no proposal cancels in agreement with the empty picture, and
lifting ⌥ re-asks at the same address. Implementation filed on the
Implementation board.

Claude-Session: https://claude.ai/code/session_014PtZdPwqZuqEDLc6wZMtEy

Kanban/UI/Board/CardFaceView.swift

@@ -249,7 +249,7 @@ struct CardFaceView: View, Equatable {
         // Constant, whether or not a stripe paints: every card's text sits on the same grid, so
         // colouring a card never shifts its title relative to its uncoloured neighbours.
         .padding(.leading, stripeWidth)
-        .background(RoundedRectangle(cornerRadius: cornerRadius).fill(.background.secondary))
+        .background(RoundedRectangle(cornerRadius: cornerRadius).fill(BoardSurface.cardPlate))
         .overlay(alignment: .leading) { accentStripe }
         // The selection treatment, which a hovering Finder file drag borrows outright: "the card
         // highlights while hovered" (04-interactions.md ▸ Drag and drop), and the accent stroke is
@@ -510,7 +510,7 @@ struct CardFaceView: View, Equatable {
             width: BoardMetrics.cardReplicaWidth(measured: measuredWidth, bodyPointSize: pointSize),
             alignment: .leading
         )
-        .background(RoundedRectangle(cornerRadius: cornerRadius).fill(.background.secondary))
+        .background(RoundedRectangle(cornerRadius: cornerRadius).fill(BoardSurface.cardPlate))
         .overlay(alignment: .leading) { accentStripe }
         .dragReplicaShadow()
     }