Implement drag & drop with the locality model

The second half: system drag sessions over phase 1's model, per
DRAG-REORDER.md and 04-interactions.md § Drag & drop.

- Card faces, lane headers, and trash rows drag as NSItemProvider sessions
  (two exported UTTypes, JSON payload in flatten order, plain-text titles as
  the secondary representation) — replacing m4's custom lane-reorder gesture
  and trash drag-out wholesale; the app-wide DragSession carries the members,
  the frozen dragged sizes, the live proposal, and the effective operation.
- Three drop delegates (lane masonry, strip, window fallback), each accepting
  both types and routing internally per the single-target-dispatch rule; the
  cursor is the physical mouse converted to strip space; proposals come from
  DropSlotMath with hysteresis threaded through, and the lane-strip proposal
  clamps in front of the shown trash.
- Locality picks the default — move within a board, copy across, the badge
  tracking live; ⌥ forces copy (ignored on within-board lane drags), ⌘
  forces move; trash rows restore within their board (positional), copy out
  across boards by default, ⌘ forcing the true restore-move.
- N contiguous shadows with reflow keyed on the proposal; the
  committed-overlay hold renders the dropped arrangement until the reload
  echo lands (1.5 s dissolution deadline for refused writes); the
  re-grounding trio: geometry re-derives per render, proposals re-validate
  by liveness at release, an emptied drag cancels itself.
- Edge autoscroll (ticking driver over DragAutoScrollMath, re-targeting per
  step), the mouse-up-gated late-event cleanup, and the polling watchdog —
  the pathfinder's lifecycle traps, ported.
- Store: moveLanes and multi-card restoreByDrag join the one-bracket drop
  commits.

784 unit tests.

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

Kanban/App/AppModel.swift

@@ -165,6 +165,17 @@ public final class AppModel {
     /// board-scoped and this list deliberately is not.
     public let styleRecents = StyleRecents()
 
+    /// The app's one drag session (DRAG-REORDER.md; 04-interactions.md ▸ Drag and drop).
+    ///
+    /// App-wide for the reason cross-board drags exist at all: **a drag crosses windows**, so the
+    /// source board hides the dragged items while any other open board's drop delegates propose a
+    /// landing spot for them. It lives here rather than as a global for `styleRecents`' reason — a
+    /// test holds its own rather than colliding with the app's — and every board window reaches it
+    /// through the environment.
+    /// Internal rather than `public`, unlike its neighbours: the drag is entirely a UI-layer
+    /// concern, and nothing outside this module has any business reaching into a gesture in flight.
+    let dragSession = DragSession()
+
     // MARK: Sessions
 
     /// One open board window and everything hanging off it.