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/lookup.ts

@@ -8,6 +8,24 @@ export interface BehaviorProgram {
   defaults: PropertyBag;
 }
 
-export type BehaviorLookup = (assetId: string) => BehaviorProgram | null;
+/** One state of a machine: the node id is what a port and the runtime hold, the name is the label. */
+export interface MachineState {
+  id: string;
+  name: string;
+}
 
-export const NO_BEHAVIORS: BehaviorLookup = () => null;
+export interface StateMachine {
+  name: string;
+  states: MachineState[];
+}
+
+/**
+ * What a node may look up in the project around its graph: the behaviors it can run and the
+ * state machines it can read. One handle, so no caller can resolve a node with half of it.
+ */
+export interface GraphLookup {
+  behavior(assetId: string): BehaviorProgram | null;
+  machine(assetId: string): StateMachine | null;
+}
+
+export const NO_LOOKUP: GraphLookup = { behavior: () => null, machine: () => null };