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/graph/NodeContext.ts

@@ -62,6 +62,11 @@ export interface NodeContext {
   /** The behavior's parent object – the one the graph acts on. */
   owner: EntityHandle | null;
   variables: PropertyBag;
+  /**
+   * The current state of each state machine this instance has set, keyed by machine asset id,
+   * as the state node's id. A machine not in here is in no state: `State branch` follows nothing.
+   */
+  states: Record<string, string>;
   /** Seconds since the world started and since the last tick. */
   time: number;
   dt: number;