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/vfs/VFS_CORE.md

@@ -418,7 +418,7 @@ await vfs.importDirectory()// Import directory from filesystem
 
 ## Bulk Write Operations
 
-**v6.5.0**: `bulkWrite` efficiently processes multiple VFS operations with automatic ordering to prevent race conditions.
+`bulkWrite` efficiently processes multiple VFS operations with automatic ordering to prevent race conditions.
 
 ```javascript
 const result = await vfs.bulkWrite([
@@ -439,7 +439,7 @@ console.log(`Successful: ${result.successful}, Failed: ${result.failed.length}`)
 - `delete` - Delete file
 - `update` - Update file metadata only
 
-**Operation ordering (v6.5.0):**
+**Operation ordering:**
 1. `mkdir` operations run **first**, sequentially, sorted by path depth (shallowest first)
 2. Other operations (`write`, `delete`, `update`) run **after** in parallel batches of 10