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/editor/debugHook.ts

@@ -56,10 +56,10 @@ export function installDebugHook(app: Application, state: EditorState, editor: E
    * *minimum* width, so a label wider than the column pushes its control right – which is why a
    * fixed offset from the label's own text is the wrong way to click one.
    */
-  const fieldOf = (label: string): { x: number; y: number; left: number } | null => {
+  const fieldOf = (label: string, last = false): { x: number; y: number; left: number } | null => {
     let found: Text | null = null;
     const walk = (node: Container): void => {
-      if (found !== null || !node.visible) return;
+      if ((found !== null && !last) || !node.visible) return;
       if (node instanceof Text && node.text === label) {
         found = node;
         return;