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/NodeRegistry.ts

@@ -81,6 +81,11 @@ export interface NodeDef {
    * offers, which values the behavior a call node names takes. `resolveDef` applies it.
    */
   derivePorts?(source: PortSource): DerivedShape;
+  /**
+   * Outputs readable at any moment, even while the node's exec input has not fired in this pulse.
+   * Never cached, so a read after a write sees the new value.
+   */
+  read?(ctx: NodeContext, params: PropertyBag): Record<string, Value>;
   /**
    * Event nodes have no `run` – the interpreter seeds their outputs. `def` is resolved for the node at
    * hand, the only way a node with derived ports knows how many it has.