refactor(core): add consumerOnSignalRead hook to ReactiveNode

This hook allows a consumer to create `ReactiveNode`s with custom behavior
when signals are read.

packages/core/primitives/signals/src/graph.ts

@@ -60,6 +60,7 @@ export const REACTIVE_NODE: ReactiveNode = {
   producerMustRecompute: () => false,
   producerRecomputeValue: () => {},
   consumerMarkedDirty: () => {},
+  consumerOnSignalRead: () => {},
 };
 
 /**
@@ -156,6 +157,11 @@ export interface ReactiveNode {
   producerMustRecompute(node: unknown): boolean;
   producerRecomputeValue(node: unknown): void;
   consumerMarkedDirty(node: unknown): void;
+
+  /**
+   * Called when a signal is read within this consumer.
+   */
+  consumerOnSignalRead(node: unknown): void;
 }
 
 interface ConsumerNode extends ReactiveNode {
@@ -185,6 +191,8 @@ export function producerAccessed(node: ReactiveNode): void {
     return;
   }
 
+  activeConsumer.consumerOnSignalRead(node);
+
   // This producer is the `idx`th dependency of `activeConsumer`.
   const idx = activeConsumer.nextProducerIndex++;