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>

tests/helpers.ts

@@ -181,6 +181,7 @@ export function fakeContext(overrides: Partial<NodeContext> = {}): FakeContext {
     self: fakeEntity('self'),
     owner: fakeEntity('owner'),
     variables: { speed: 100 },
+    states: {},
     time: 0,
     dt: 0.5,
     logs,
@@ -213,9 +214,10 @@ export function runDef(
   ctx: NodeContext,
   inputs: Record<string, Value>,
   params: PropertyBag = {},
+  entry = 'exec',
 ): RunResult {
   const def = requireNodeDef(type);
-  return def.run!(ctx, inputs, params, def);
+  return def.run!(ctx, inputs, params, def, entry);
 }
 
 /** Runs one node definition in isolation and returns its data outputs. */