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/core/graph/nested.ts

@@ -69,12 +69,29 @@ export function resolveDef(node: GraphNode, lookup: BehaviorLookup): NodeDef | u
 }
 
 /**
- * Everything that changes a node's ports, as one string. Views and the inspector rebuild when it
+ * Why this call cannot do anything, or null if it can. A call is the one node that depends on a
+ * *different* graph, so it is the one node that can look perfectly healthy and still be dead.
+ */
+export function callProblem(node: GraphNode, lookup: BehaviorLookup, ownerId = ''): string | null {
+  if (node.type !== RUN_BEHAVIOR) return null;
+  const id = String(node.params[BEHAVIOR_PARAM] ?? '');
+  if (!id) return 'No behavior picked';
+  if (id === ownerId) return 'A behavior cannot run itself';
+  const program = lookup(id);
+  if (!program) return 'That behavior is gone';
+  if (!terminalNode(program.graph, BEHAVIOR_START)) return 'No start node – does nothing';
+  return null;
+}
+
+/**
+ * Everything that changes how a node looks, as one string. Views and the inspector rebuild when it
  * changes – a port list that is derived rather than fixed has no other way of being noticed.
  */
-export function portSignature(node: GraphNode, lookup: BehaviorLookup): string {
+export function portSignature(node: GraphNode, lookup: BehaviorLookup, ownerId = ''): string {
   const def = resolveDef(node, lookup);
   if (!def) return 'missing';
   const list = (ports: PortDef[]) => ports.map((p) => `${p.key}:${p.type}`).join(',');
-  return `${node.type}|${def.label}|${list(def.inputs)}|${list(def.outputs)}`;
+  return `${node.type}|${def.label}|${list(def.inputs)}|${list(def.outputs)}|${
+    callProblem(node, lookup, ownerId) ?? ''
+  }`;
 }