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/06-graph.mjs

@@ -34,7 +34,7 @@ export async function graph(t) {
   };
   const variableKeys = async () => (await asset('mover')).variables.map((v) => v.key).join(',');
 
-  // 6a. Selecting the Mover asset binds the graph; the Graph tab shows it.
+  // 6a. Selecting the Mover asset binds the graph; the Behavior tab shows it.
   await clickText('Mover');
   await evaluate(`__gm.showTab('graph')`);
   await sleep(250);