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/shared.ts

@@ -1,7 +1,8 @@
-import type { PropertyValue } from '../../model/Properties';
+import { type PropertyBag, type PropertyValue, textOf } from '../../model/Properties';
 import type { PropertyDef } from '../../properties/PropertySchema';
-import type { NodeDef, PortDef } from '../NodeRegistry';
-import { asBoolean, asNumber } from '../values';
+import type { NodeContext } from '../NodeContext';
+import type { DerivedShape, NodeDef, PortDef, PortSource, RunResult } from '../NodeRegistry';
+import { asBoolean, asNumber, type Value, writeTyped } from '../values';
 
 export const exec = (key = 'exec', label = ''): PortDef => ({ key, label, type: 'exec' });
 export const num = (key: string, label = key): PortDef => ({ key, label, type: 'number' });
@@ -33,6 +34,31 @@ export const variableParam: PropertyDef = {
   options: [],
 };
 
+const variableKey = (params: PropertyBag) => textOf(params.variable ?? '');
+
+export const readVariable = (ctx: NodeContext, params: PropertyBag): Record<string, Value> => ({
+  value: ctx.variables[variableKey(params)] ?? null,
+});
+
+export function writeVariable(
+  ctx: NodeContext,
+  inputs: Record<string, Value>,
+  params: PropertyBag,
+): RunResult {
+  const key = variableKey(params);
+  if (key) writeTyped(ctx.variables, key, inputs.value);
+  return { next: 'exec' };
+}
+
+/** Once a variable is picked, the title names it: "Get speed", or just "speed". */
+export const variableTitle =
+  (verb: string) =>
+  ({ node }: PortSource): DerivedShape => {
+    const name = variableKey(node.params);
+    if (!name) return {};
+    return { label: verb ? `${verb} ${name}` : name };
+  };
+
 /** No `assetTypes`: any placeable asset is a valid answer here. */
 export const assetParam: PropertyDef = { key: 'asset', label: 'Asset', kind: 'asset', default: '' };