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/api/DataAPI.ts

@@ -210,7 +210,7 @@ export class DataAPI {
               this.validateImportItem(mapped)
             }
 
-            // v4.0.0: Save entity - separate vector and metadata
+            // Save entity - separate vector and metadata
             const id = mapped.id || this.generateId()
             const noun: HNSWNoun = {
               id,