Make a call node say why it does nothing, and easier to place

A behavior node that cannot run looked exactly like a healthy one: no
start node, no behavior picked, a deleted behavior or a self-call were
all silently skipped. The call still had to be forgiving at runtime -- a
dangling reference must not abort a game -- so the fix is diagnosis, not
an exception: describeProblem() names the one reason, the node draws it
in red under its title, and the inspector repeats it where you pick the
behavior. A missing end node is deliberately not a problem: a call is
always synchronous, and the end node only decides which values come back.

Two ways in: a behavior dragged from the asset panel lands in the graph
under the pointer, and double-clicking a call node opens the graph it
runs. Both reuse machinery that already existed -- the asset panel
already starts the drag, and list views already report double clicks.

The e2e module needed its assets at the head of the asset list: after 18
modules the list is long enough that a newly appended row scrolls out of
view, and the press then lands on the panel behind it, starting no drag
at all. That looked exactly like a rejected drop and cost two full runs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/editor/debugHook.ts

@@ -1,7 +1,7 @@
 import { type Application, Color, type ColorSource, type Container, Rectangle, Text } from 'pixi.js';
 import { resolveObjects } from '../core/model/instance';
 import { findObjectHolder } from '../core/model/Project';
-import { Button, ui, Widget } from '../ui';
+import { activeDrag, Button, ui, Widget } from '../ui';
 import { textureLibrary } from '../viewport/TextureLibrary';
 import type { CentreTab, Editor } from './Editor';
 import { currentImagePicker } from './texture/ImagePicker';
@@ -169,6 +169,8 @@ export function installDebugHook(app: Application, state: EditorState, editor: E
     /** Where a dragged asset would land in the Scene tab, and why not when it may not. */
     dropPreview: () => viewport.dropInfo(),
     /** An asset row by id – two assets can share a name with the container picker's label. */
+    /** The panel-to-view drag in flight, if any – a drop that never started looks like a refused one. */
+    dragPayload: () => activeDrag(),
     assetRow: (id: string) => assets.rowGlobal(id),
     assetRowColor: (id: string) => assets.rowColor(id),
     gameInfo: () => gameView.debugInfo(),
@@ -185,6 +187,7 @@ export function installDebugHook(app: Application, state: EditorState, editor: E
         graphEditor.canvas.portGlobal(nodeId, port, isOutput),
       nodeAt: (nodeId: string) => graphEditor.canvas.nodeGlobal(nodeId),
       nodeLabel: (nodeId: string) => graphEditor.canvas.nodeLabel(nodeId),
+      nodeProblem: (nodeId: string) => graphEditor.canvas.nodeProblem(nodeId),
       /** Drawn rectangles on screen: what "fit" has to keep inside `canvasRect`. */
       nodeRect: (nodeId: string) => graphEditor.canvas.nodeRect(nodeId),
       canvasRect: () => graphEditor.canvas.canvasRect(),