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.

docs/guides/standard-import-progress.md

@@ -65,7 +65,7 @@ interface ImportProgress {
   /** Estimated time remaining (milliseconds) */
   eta?: number
 
-  /** Whether data is queryable at this point (v4.2.0+) */
+  /** Whether data is queryable at this point */
   queryable?: boolean
 }
 ```