Let a Start input or a variable name the state machine it carries

A sub-behavior that gets its machine through a Start input had no
state ports on Is in state, Set state and the other state nodes.
Their ports came only from an inline pick or a State machine literal,
so a machine handed in from outside left them empty.

- A statemachine port of a Start node can now name a machine
  (DeclaredPort.machine). The inspector shows the pick next to the
  port name. SetPortMachineCommand sets it with undo.
- machineIdOf follows an edge from such a port to that machine, so
  the state nodes it feeds offer its states.
- A Run behavior node has the machine picked in advance on that input.
- A different pick there is allowed, but the node warns, because the
  called graph asks for the states of its own machine.
- A statemachine variable of a Variables node can name a machine
  the same way, picked next to the variable's row.
- The inspector signature of a node now includes the picked machine.
  Before, the pick changed the ports but the dropdown kept its text
  until the node was selected again.
- A local machine handed in still starts in no state every call.
  That rule from #229 is unchanged.

Tests:
- tests/core/start-machine.test.ts (7): ports with and without a
  machine and undo, save and load, the preset value moving a shared
  machine through the call, the warning, a port of another kind,
  the inspector signature, a Variables node.
- Counter checks: without the new branch in machineIdOf 2 tests
  fail, without the machine in the signature 1 test fails.
- Gate on melektaus-atx: 239 files, 3599 tests, CHECK EXIT 0.

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

src/core/store/serialize.ts

@@ -480,7 +480,11 @@ function normalizePorts(raw: unknown, fixed: readonly string[], writable: boolea
     const kind = (DECLARED_PORT_KINDS as readonly string[]).includes(String(p.kind))
       ? (p.kind as DeclaredPortKind)
       : 'number';
-    ports.push(writable && p.write === true ? { key, kind, write: true } : { key, kind });
+    const port: DeclaredPort = writable && p.write === true ? { key, kind, write: true } : { key, kind };
+    // A machine that is gone is kept, as a struct type is: the port then offers no states.
+    if (kind === 'statemachine' && typeof p.machine === 'string' && p.machine !== '')
+      port.machine = p.machine;
+    ports.push(port);
   }
   return ports;
 }