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/types/graphTypes.ts

@@ -303,7 +303,7 @@
  * }
  * ```
  *
- * ## Stage 3 Changes from v5.4.0
+ * ## Stage 3 Changes
  *
  * ### Nouns Added (+11)
  * agent, quality, timeInterval, function, proposition, socialGroup, institution,
@@ -323,7 +323,7 @@
  * createdBy (use inverse of creates), supervises (use inverse of reportsTo)
  *
  * **Net Change**: +9 nouns (31 → 40), +48 verbs (40 → 88) = +57 types total
- * **Coverage**: 60% (v5.4.0) → 95% (Stage 3)
+ * **Coverage**: 60% → 95% (Stage 3)
  */
 
 // Common metadata types