Spawn node: place assets at runtime into the play copy, Spawner demo behavior

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

README.md

@@ -31,7 +31,9 @@ Scene tab shows the live positions (symbols, gizmos and dragging included), the
 inspector show and edit the running objects, the graph can be changed, undo/redo work on the
 play session's own history. All of it happens in the runtime copy: values a behavior changes
 appear as overrides (bold, ↺ resets to the asset value), objects a graph destroys vanish from
-the lists, and Esc/Stop brings back exactly what was saved. Autosave pauses while playing.
+the lists, objects a graph spawns (`Spawn` node) appear in them – selectable, editable, still
+only in the runtime copy –, and Esc/Stop brings back exactly what was saved. Autosave pauses
+while playing.
 
 ## Behaviors (node graphs)
 
@@ -54,8 +56,8 @@ shown on the node and editable in the inspector.
   variable`, `Get/Set property`, `Get position` (local and world), literals, `Add/Subtract/Multiply/Divide`,
   `Min/Max/Abs/Sqrt/Clamp`, `Length`, `Distance`, `Normalize`, `Axis input` (four keys → a
   normalized direction, so diagonals are not faster), `Compare`, `And/Or/Not`, `Key is down`, `Is touching` ·
-  actions `Move` (dx, dy, scale, dt – connect an event's dt for per-second movement), `Set position`, `Destroy`,
-  `Log`, `Stop game`. A node type is one `registerNode()` call; palette, ports, validation and
+  actions `Move` (dx, dy, scale, dt – connect an event's dt for per-second movement), `Set position`, `Spawn`
+  (asset setting + x/y/z, optional parent → outputs the new object), `Destroy`, `Log`, `Stop game`. A node type is one `registerNode()` call; palette, ports, validation and
   inspector fields are generated from it. A port may declare a `default` for its inline value
   (e.g. `Move.scale = 1`), shown on the node and in the inspector.
 - **Idiomatic movement** (the demo's "Mover", five nodes): `On update → Move(Owner, dx = Axis