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/GraphInterpreter.ts

@@ -96,7 +96,7 @@ export class GraphInterpreter {
     const inputs = this.collectInputs(node, ctx);
     const result = def.run(ctx, inputs, node.params, def);
     const returned = result.call ? this.call(result.call, ctx, depth) : {};
-    this.frame.outputCache.set(node.id, { ...result.outputs, ...returned });
+    if (!def.read) this.frame.outputCache.set(node.id, { ...result.outputs, ...returned });
     this.trace?.onNode(node.id);
     if (result.repeat) this.loop(node, result.repeat, ctx, depth);
     const next = result.next === undefined ? [] : Array.isArray(result.next) ? result.next : [result.next];
@@ -167,6 +167,10 @@ export class GraphInterpreter {
     if (!source) return null;
     const def = this.def(source);
     if (!def) return null;
+    if (def.read) {
+      this.trace?.onNode(nodeId);
+      return def.read(ctx, source.params)[port] ?? null;
+    }
     // Exec nodes that have not run in this pulse have no value yet.
     if (isEventNode(def) || hasExecInput(def) || !def.run) return null;
     if (++this.steps > MAX_STEPS_PER_FIRE)