fix: exclude __words__ keyword index from corruption detection and getStats()

The __words__ keyword index stores 50-5000 entries per entity (one per
word), which inflated avg entries/entity well above the corruption
threshold of 100. This caused:

1. validateConsistency() to falsely detect corruption on every startup,
   triggering unnecessary clearAllIndexData() + rebuild() cycles
2. getStats() to log false "Metadata index may be corrupted" warnings
   and report inflated totalEntries/totalIds stats

Both methods now skip __words__ when counting, so stats and health
checks reflect metadata fields only (noun, type, createdAt, etc.).
Keyword search is unaffected since the __words__ field index itself
is not modified.

src/utils/periodicCleanup.ts

@@ -215,7 +215,7 @@ export class PeriodicCleanup {
       
       for (const noun of nounsResult.items) {
         try {
-          // v4.0.0: Cast NounMetadata to NamespacedMetadata for isDeleted check
+          // Cast NounMetadata to NamespacedMetadata for isDeleted check
           if (!noun.metadata || !isDeleted(noun.metadata as any)) {
             continue // Not deleted, skip
           }