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/graphLookup.ts

@@ -0,0 +1,22 @@
+import type { GraphLookup } from '../graph/lookup';
+import { statesOf } from '../graph/stateMachine';
+import { resolveAssetProps } from '../properties/resolve';
+import { findAssetOf, type Project } from './Project';
+
+/**
+ * What a graph may see of the project around it. A behavior's `defaults` are its variable values;
+ * every call copies them, so a nested behavior never carries state from one call to the next.
+ */
+export function graphLookup(project: Project): GraphLookup {
+  return {
+    behavior: (assetId) => {
+      const asset = findAssetOf(project, assetId, 'behavior');
+      if (!asset) return null;
+      return { name: asset.name, graph: asset.graph, defaults: resolveAssetProps(asset) };
+    },
+    machine: (assetId) => {
+      const asset = findAssetOf(project, assetId, 'statemachine');
+      return asset ? { name: asset.name, states: statesOf(asset.graph) } : null;
+    },
+  };
+}