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/collapsed-node.test.ts

@@ -4,7 +4,7 @@ import { CollapseNodesCommand } from '../../src/core/commands/graphCommands';
 import type { BehaviorAsset } from '../../src/core/model/Asset';
 import { findAsset } from '../../src/core/model/Project';
 import { deserializeProject, serializeProject } from '../../src/core/store/serialize';
-import { foldMarkKey } from '../../src/editor/graph/folding';
+import { collapsedNodeWidth, foldMarkKey } from '../../src/editor/graph/folding';
 import { graphEditor, makeBehavior, makeProject, roundTrip } from '../helpers';
 
 function threeNodes() {
@@ -61,6 +61,16 @@ describe('collapsing nodes', () => {
   });
 });
 
+describe('the width of a collapsed node', () => {
+  it('fits the title plus its chrome, rounded up so the title is not cut', () => {
+    expect(collapsedNodeWidth(40.2, 31, 176)).toBe(72);
+  });
+
+  it('never grows past an open node – a long title is cut instead', () => {
+    expect(collapsedNodeWidth(300, 31, 176)).toBe(176);
+  });
+});
+
 describe('hidden links at a collapsed node', () => {
   it('share one mark per side, because every port sits on the same point', () => {
     const a = foldMarkKey({ node: 'n', port: 'x' }, false, true);