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>

tests/core/node-catalog.test.ts

@@ -150,6 +150,22 @@ const CASES: Record<string, Case[]> = {
       after: (ctx) => expect(ctx.variables).toEqual({ speed: 100 }),
     },
   ],
+  // Reading without a flow needs the interpreter: see variable-node.test.ts.
+  Variable: [
+    {
+      name: 'writes its variable when the flow arrives, keeping the declared type',
+      inputs: { value: '250' },
+      params: { variable: 'speed' },
+      next: 'exec',
+      after: (ctx) => expect(ctx.variables.speed).toBe(250),
+    },
+    {
+      name: 'no variable chosen writes nothing',
+      inputs: { value: 5 },
+      next: 'exec',
+      after: (ctx) => expect(ctx.variables).toEqual({ speed: 100 }),
+    },
+  ],
 
   // references and properties