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/setup.ts

@@ -1,7 +1,7 @@
 /**
  * Brainy Setup - Minimal Polyfills
  *
- * ARCHITECTURE (v7.0.0):
+ * ARCHITECTURE:
  * Brainy uses Candle WASM (Rust-based) for embeddings.
  * No transformers.js or ONNX Runtime dependency, no hacks required.
  *