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

WISHLIST.md

@@ -23,3 +23,7 @@ The card window's History section (DESIGN/05-card-window.md) is read-only in v1.
 ### 5. Lane history
 
 Per-lane commit trail, the lane sibling of the card History section. Undecided whether it's wanted at all (a lane's own commits — rename, restyle, resize, reorder — are sparse; most of what changes "in" a lane belongs to its cards) and where it would surface, since lanes have no window.
+
+### 6. EchoLedger-synthesized foreign undo (base edition)
+
+Narrow base Lanework's honest gap vs Pro — foreign/agent edits aren't ⌘Z-able without git (DESIGN/13-native-undo.md ▸ Out of scope). The EchoLedger already classifies every foreign diff for announcements; it could synthesize inverse operations and push foreign steps onto the native stack, making an agent's edit undoable in base too. Real design needed: ordering against app steps, attribution in menu titles, user expectations when undoing work they didn't do, interaction with the staleness rules.