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/nodes/index.ts

@@ -26,7 +26,14 @@ import { IsTouching } from './data/IsTouching';
 import { Join } from './data/Join';
 import { KeyIsDown } from './data/KeyIsDown';
 import { Length } from './data/Length';
-import { BooleanLiteral, ListLiteral, NumberLiteral, StringLiteral, TextureLiteral } from './data/literals';
+import {
+  BooleanLiteral,
+  ListLiteral,
+  NumberLiteral,
+  StateMachineLiteral,
+  StringLiteral,
+  TextureLiteral,
+} from './data/literals';
 import { Max } from './data/Max';
 import { Min } from './data/Min';
 import { Modulo } from './data/Modulo';
@@ -76,6 +83,9 @@ import { Branch } from './flow/Branch';
 import { Repeat } from './flow/Repeat';
 import { RunBehavior } from './flow/RunBehavior';
 import { Sequence } from './flow/Sequence';
+import { SetState } from './flow/SetState';
+import { StateBranch } from './flow/StateBranch';
+import { State } from '../stateMachine';
 import { Variables } from './variable/Variables';
 
 /**
@@ -99,6 +109,8 @@ export const BUILT_IN_NODES: readonly NodeDef[] = [
   Sequence,
   Repeat,
   RunBehavior,
+  StateBranch,
+  SetState,
   BehaviorEnd,
   Variables,
   Self,
@@ -114,6 +126,7 @@ export const BUILT_IN_NODES: readonly NodeDef[] = [
   BooleanLiteral,
   StringLiteral,
   TextureLiteral,
+  StateMachineLiteral,
   Add,
   Subtract,
   Multiply,
@@ -168,3 +181,5 @@ export const BUILT_IN_NODES: readonly NodeDef[] = [
 ];
 
 for (const def of BUILT_IN_NODES) registerNode(def);
+// Registered so a machine's graph loads and draws, but not built in: no behavior graph offers it.
+registerNode(State);