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/Info.plist

@@ -44,6 +44,30 @@
 				<string>kanban</string>
 			</dict>
 		</dict>
+		<!-- The drag session payload types (DRAG-REORDER.md; 04-interactions.md ▸ Drag and drop).
+		     Same-app transfer is the only real consumer — both boards are open in this app — but a
+		     system drag session is what crosses window boundaries, draws the copy badge and gives
+		     the full-size replica, and a system session needs a declared type to carry. -->
+		<dict>
+			<key>UTTypeIdentifier</key>
+			<string>dev.rzen.indie.kanban.cards</string>
+			<key>UTTypeConformsTo</key>
+			<array>
+				<string>public.data</string>
+			</array>
+			<key>UTTypeDescription</key>
+			<string>Lanework Cards</string>
+		</dict>
+		<dict>
+			<key>UTTypeIdentifier</key>
+			<string>dev.rzen.indie.kanban.lanes</string>
+			<key>UTTypeConformsTo</key>
+			<array>
+				<string>public.data</string>
+			</array>
+			<key>UTTypeDescription</key>
+			<string>Lanework Lanes</string>
+		</dict>
 	</array>
 	<key>CFBundleDocumentTypes</key>
 	<array>