Let a nested behavior without a start node run from On start (#230)

A behavior built as a plain behavior, with On start and no Behavior
start, did nothing when another behavior called it through Run
behavior. The node only showed a warning. Such a behavior should run
as a call too, without the author having to rebuild its entry.

- New callEntries() in nested.ts returns the Behavior start node or,
  when there is none, every On start node. callProblem uses it too.
- GraphInterpreter.call() follows all entries one after the other,
  in graph order, in the same frame and with the same variables.
- Behavior end still returns what reaches it. A call through On start
  has no inputs, because only the start node declares them.
- It is a fallback only. With a Behavior start present, On start
  nodes stay out of the call.
- The node warning now reads "No start or On start node - does
  nothing" and only shows when neither exists.
- Keep and local state machines behave as before.

Tests:
- ./check.sh: 163 files, 2396 tests, CHECK EXIT 0 (before 2392).
- 4 new unit tests in tests/core/nested-behavior.test.ts: the flow
  after On start runs, the return value arrives, several On start
  nodes run in graph order, a start node keeps On start out.
- e2e 19: ALL PASSED, 35 checks. Only the expected warning text
  changed, the check sum stays at 1385.
- Counter-checks: without the fallback 3 tests fail. With only the
  first On start exactly the order test fails.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/core/graph/nested.ts

@@ -20,6 +20,7 @@ export const BEHAVIOR_END = 'BehaviorEnd';
 export const RUN_BEHAVIOR = 'RunBehavior';
 export const VARIABLES = 'Variables';
 export const PROPERTIES = 'Properties';
+export const ON_START = 'OnStart';
 
 export const BEHAVIOR_PARAM = 'behavior';
 export const VARIABLES_PARAM = 'variablesAsset';
@@ -92,6 +93,15 @@ export function terminalNode(graph: Graph, type: string): GraphNode | undefined
   return graph.nodes.find((n) => n.type === type);
 }
 
+/**
+ * Where a call into `graph` begins: its start node, or – for a behavior that has none – every
+ * `On start` in it, in graph order, as when the behavior starts on its own (#230).
+ */
+export function callEntries(graph: Graph): GraphNode[] {
+  const start = terminalNode(graph, BEHAVIOR_START);
+  return start ? [start] : graph.nodes.filter((n) => n.type === ON_START);
+}
+
 export function isTerminalType(type: string): boolean {
   return type === BEHAVIOR_START || type === BEHAVIOR_END;
 }
@@ -267,7 +277,7 @@ export function callProblem(node: GraphNode, lookup: GraphLookup, ownerId = ''):
   if (id === ownerId) return 'A behavior cannot run itself';
   const program = lookup.behavior(id);
   if (!program) return 'That behavior is gone';
-  if (!terminalNode(program.graph, BEHAVIOR_START)) return 'No start node – does nothing';
+  if (callEntries(program.graph).length === 0) return 'No start or On start node – does nothing';
   return null;
 }