State machines: asset, tab, State branch and Set state nodes (#51)

A state machine is an asset whose graph holds one `State` node per state
(no ports, name as title), edited in a new "State machine" tab that reuses
the graph canvas: right-click places "State n", snap toggle shared with the
Behavior tab via SnapToggle. `statemachine` is a value kind like texture
(port, literal node, variable, struct field, tagged list item).

Behaviors use it through `State branch` (one exec output per state) and
`Set state` (one exec input per state; the interpreter now hands `run` the
entry port). States live per behavior instance in NodeContext.states, with
no initial state. Ports derive from the inline picker or a wired
`State machine` literal; BehaviorLookup became GraphLookup with
behavior(id) and machine(id).

The Graph tab is labelled "Behavior" (key unchanged). Unit tests
(state-machine, kinds, catalog, ports), e2e step 26 and README added.

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

src/core/model/Project.ts

@@ -1,4 +1,11 @@
-import { type Asset, type ContainerAsset, isSceneAsset, type SceneAsset } from './Asset';
+import {
+  type Asset,
+  type ContainerAsset,
+  type GraphAsset,
+  isGraphAsset,
+  isSceneAsset,
+  type SceneAsset,
+} from './Asset';
 
 /** 2: a placed container carries no copy of its template. */
 export const PROJECT_FORMAT_VERSION = 2;
@@ -38,6 +45,11 @@ export function listScenes(project: Project): SceneAsset[] {
   return project.assets.filter(isSceneAsset);
 }
 
+export function findGraphAsset(project: Project, assetId: string): GraphAsset | undefined {
+  const asset = findAsset(project, assetId);
+  return asset && isGraphAsset(asset) ? asset : undefined;
+}
+
 /**
  * Everything that holds placed objects. A container's `objects` are a template, so anything
  * walking "all placed objects" must cover both or it silently skips the templates.