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/import-quick-reference.md

@@ -91,7 +91,7 @@ await brain.import(file, {
 })
 ```
 
-### Import Tracking (v4.10.0+)
+### Import Tracking
 
 Track and organize imports by project:
 
@@ -149,7 +149,7 @@ await brain.import(file, {
 })
 ```
 
-### Always-On Streaming (v4.2.0+)
+### Always-On Streaming
 
 All imports use streaming with adaptive flush intervals. Query data as it's imported: