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/BoardView.swift

@@ -423,8 +423,9 @@ struct BoardView: View {
     /// hand-written hex, they reach the same place — has its text colour computed against the
     /// threshold by `BoardTextInk`, composited over the window background in the active appearance
     /// and recomputed on an appearance flip; the two subtrees that sit on this fill
-    /// (`LaneView.header` and `TrashLaneView.header` — every other surface on the board carries its
-    /// own opaque plate) take the answer as a `\.colorScheme` override.
+    /// (`LaneView.header` and `TrashLaneView.header` — their plates are translucent washes the
+    /// colour shows through, where every card carries its own opaque plate,
+    /// `BoardSurface.cardPlate`) take the answer as a `\.colorScheme` override.
     ///
     /// **One path, two verification stories** (`ContrastMath`): the twelve palette pairs are checked
     /// statically, by a test over the ink this seam chooses for each of them (03-board-ui.md §