Add the Variable node and show the variable name in node titles

Get/Set variable nodes now title themselves "Get <name>" / "Set <name>", and
the new Variable node just "<name>"; the title is the same folded or open,
and a collapsed node shrinks to it. The redundant "Variable: <name>" row is
gone unless the variable was deleted ("<name>?" is the only hint then).

The Variable node sets its value when an exec flow arrives and exposes the
value at any time. Its output is read when a downstream node pulls it, not
frozen at the write, so count = count + 1 is one Variable node plus an Add.
Get variable keeps reading once per pulse: switching it would change the
behaviour of existing games.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/graph/nodes/index.ts

@@ -85,6 +85,7 @@ import { GetVariable } from './variable/GetVariable';
 import { GetVariables } from './variable/GetVariables';
 import { SetVariable } from './variable/SetVariable';
 import { SetVariables } from './variable/SetVariables';
+import { Variable } from './variable/Variable';
 
 /**
  * Importing this module registers the built-in nodes. The palette sorts by category and label, so
@@ -110,6 +111,7 @@ export const BUILT_IN_NODES: readonly NodeDef[] = [
   RunBehavior,
   BehaviorEnd,
   // variables
+  Variable,
   GetVariable,
   GetVariables,
   SetVariable,