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/nodes/flow/RunBehavior.ts

@@ -4,16 +4,17 @@ import { BEHAVIOR_END, BEHAVIOR_PARAM, BEHAVIOR_START, declaredPorts, RUN_BEHAVI
 import { exec } from '../shared';
 
 /**
- * Runs another behavior's graph in place, from its start node to its end node. Its ports are those
- * the named behavior declares; the interpreter does the call, because only it can hold a second
- * graph's pulse.
+ * Runs another behavior's graph in place, from its start node (or its `On start` nodes, #230) to
+ * its end node. Its ports are those the named behavior declares; the interpreter does the call,
+ * because only it can hold a second graph's pulse.
  */
 export const RunBehavior: NodeDef = {
   type: RUN_BEHAVIOR,
   label: 'Run behavior',
   category: 'flow',
   description:
-    'Runs another behavior as a step of this one, passing values in and taking results out. A ' +
+    'Runs another behavior as a step of this one, passing values in and taking results out. It ' +
+    'begins at the Behavior start node, or at On start if that behavior has none. A ' +
     'variable marked Keep in that behavior carries on from the last call of this node on this ' +
     'object; the others start at their defaults every time.',
   inputs: [exec()],