Drag a pane's toolbar onto another pane to swap them

New interaction: click-and-drag any pane's toolbar onto another pane
to swap their positions in the tree. The shells / scrollback stay
intact (each leaf keeps its data; only the tree slot it occupies
changes).

Implementation:
- tree.ts: `swapLeaves(root, idA, idB)` walks the tree once,
  substituting one leaf for the other at each occurrence. The leaf
  objects themselves carry their id/distro/cwd/label/broadcast across,
  so React preserves the LeafPane instances via the flat-list keying.

- orchestration.tsx: add drag lifecycle to the context —
  dragSourceId / dragOverId (reactive) plus beginHeaderDrag,
  setHeaderDragOver, endHeaderDrag (stable methods).

- App.tsx: implement those methods. endHeaderDrag(true) swaps if
  source and over are different leaves.

- LeafPane.tsx: pointerdown on .pane-toolbar (skipped if the target
  is a button/input). 5px movement threshold before drag commits to
  prevent accidental swaps when clicking a chip etc. Pointer-capture
  the toolbar so we keep getting move events even outside it. Use
  document.elementFromPoint to find the leaf under the cursor.

- CSS: source pane fades to 40% opacity during drag; target pane
  shows a 3px dashed blue outline; toolbar shows grab/grabbing
  cursors.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/layout/orchestration.tsx

@@ -29,6 +29,14 @@ export interface Orchestration {
   registerPaneId: (leafId: NodeId, paneId: PaneId | null) => void;
   broadcastFrom: (originLeafId: NodeId, dataB64: string) => void;
   notify: (message: string) => void;
+
+  // Drag-header-to-swap. dragSourceId / dragOverId are reactive so leaves
+  // can apply hover/source styling. The lifecycle methods are stable.
+  dragSourceId: NodeId | null;
+  dragOverId: NodeId | null;
+  beginHeaderDrag: (leafId: NodeId) => void;
+  setHeaderDragOver: (leafId: NodeId | null) => void;
+  endHeaderDrag: (commitSwap: boolean) => void;
 }
 
 const OrchestrationContext = createContext<Orchestration | null>(null);