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/editor/graph/SnapToggle.ts

@@ -0,0 +1,17 @@
+import { Checkbox, Label, type Widget } from '../../ui';
+import type { GraphCanvas } from './GraphCanvas';
+
+/** The "Snap" checkbox every tab on the graph canvas carries in its header. */
+export class SnapToggle {
+  private readonly box = new Checkbox();
+  readonly widgets: readonly Widget[];
+
+  constructor(canvas: () => GraphCanvas) {
+    this.box.onChange = (on) => canvas().setSnapping(on);
+    this.widgets = [new Label('Snap', { style: 'muted' }), this.box];
+  }
+
+  setEnabled(on: boolean): void {
+    this.box.setEnabled(on);
+  }
+}