core: wait for prefetcher before reading PrefetchReadTimes

Forward prefetchStateReader.Wait() through *reader.WaitPrefetch and call
it before reading the read-time atomics. Eliminates the edge-case where
prefetcher goroutines outlast block execution + commit. For slow blocks
(the metric's target audience) this is a no-op; for fast blocks it
ensures the metric is complete rather than slightly under.

core/blockchain.go

@@ -683,6 +683,9 @@ func (bc *BlockChain) processBlockWithAccessList(parentRoot common.Hash, block *
 
 	// Sum read times across per-tx execution, BAL state-transition, and
 	// prefetcher async fetches. Sum-of-CPU-time, not wall-clock.
+	if w, ok := prefetchReader.(interface{ WaitPrefetch() }); ok {
+		w.WaitPrefetch() // ensure all prefetcher reads are accounted for
+	}
 	var prefetchAccountReads, prefetchStorageReads time.Duration
 	if pr, ok := prefetchReader.(interface {
 		PrefetchReadTimes() (time.Duration, time.Duration)

core/state/reader.go

@@ -576,3 +576,11 @@ func (r *reader) PrefetchReadTimes() (account, storage time.Duration) {
 	}
 	return 0, 0
 }
+
+// WaitPrefetch blocks until the wrapped prefetcher (if any) finishes its
+// task list. No-op for non-prefetch readers.
+func (r *reader) WaitPrefetch() {
+	if pr, ok := r.StateReader.(interface{ Wait() error }); ok {
+		_ = pr.Wait()
+	}
+}