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/data/literals.ts

@@ -30,6 +30,11 @@ export const TextureLiteral = literal(
   'texture',
   'A constant picture: a texture, one of its frames or an animation, as a sprite wears it.',
 );
+export const StateMachineLiteral = literal(
+  'StateMachine',
+  'statemachine',
+  'A constant state machine, for a "State branch" or a "Set state" to work on.',
+);
 export const ListLiteral = literal(
   'List',
   'list',