Implement Finder file drops

Files from Finder land on the board per 04-interactions.md § Drag & drop:

- Dropped on a card, they copy into its attachments/ (any type, multi-file),
  the face highlighting while hovered; the hovered card is resolved by
  hit-testing the same analytic masonry frames the card zones are built
  from, so attach-beats-create adds no drop region and cannot drift from
  the dispatch.
- Dropped on lane empty space, one card per file — filename minus extension
  as the title (a blank stem omits the key), fresh GUID, rank at the drop
  position through the ordinary insertion machinery, the file attached —
  all in one bracket; a failed import removes the just-minted card, so
  creating-then-abandoning never leaves an empty card behind.
- Every board drop surface now declares .fileURL beside the two board
  types (the single-target-dispatch rule); tombstoned surfaces are inert;
  file sessions ride a distinct session mode with their own watchdog and
  no hysteresis, leaving the board-drag machinery untouched.
- Also: four empty fixture directories pinned with .keep files so git
  preserves them, and a test-only visibility fix in DragSessionTests.

811 unit tests (12 new).

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

Kanban/UI/Board/BoardView.swift

@@ -125,9 +125,10 @@ struct BoardView: View {
             .onGeometryChange(for: CGRect.self) { $0.frame(in: .global) } action: { laneDrops.stripFrame = $0 }
             // **The strip's drop target** — the backdrop, the gaps, the outer margin, and the trash
             // column's footprint, which is never a landing spot of its own (04-interactions.md ▸ The
-            // trash) and so simply falls through to here. It accepts *both* board types and routes
-            // internally, because single-target dispatch has no fall-through (DRAG-REORDER.md).
-            .onDrop(of: boardDragTypes, delegate: StripDropDelegate(context: dropContext))
+            // trash) and so simply falls through to here. It accepts *every* session type — ours and
+            // external Finder file drags alike — and routes internally, because single-target
+            // dispatch has no fall-through (DRAG-REORDER.md).
+            .onDrop(of: boardDropTypes, delegate: StripDropDelegate(context: dropContext))
         }
         .background(boardBackground)
         // The window-level fallback, *behind* the specific targets: a release over any in-window
@@ -135,7 +136,7 @@ struct BoardView: View {
         // cancel-snapback — the drop lands where the shadows show, which is what the shadows promise.
         .background {
             Color.clear
-                .onDrop(of: boardDragTypes, delegate: BoardFallbackDropDelegate(context: dropContext))
+                .onDrop(of: boardDropTypes, delegate: BoardFallbackDropDelegate(context: dropContext))
         }
         // **The committed-overlay hold's hand-off** (DRAG-REORDER.md § The committed-overlay hold):
         // the overlay stands in for an arrangement that is on disk but not yet in the snapshot, and