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>

tools/e2e/index.mjs

@@ -28,6 +28,7 @@ import { structVariable } from './22-struct.mjs';
 import { variableNodes } from './23-variable-nodes.mjs';
 import { propertyNodes } from './24-property-nodes.mjs';
 import { animation } from './25-animation.mjs';
+import { stateMachine } from './26-state-machine.mjs';
 
 export const STEPS = [
   hierarchy,
@@ -55,4 +56,5 @@ export const STEPS = [
   variableNodes,
   propertyNodes,
   animation,
+  stateMachine,
 ];