Spec: restore the blank line before the section rule

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/Architecture-Build-Spec.md

@@ -1443,6 +1443,7 @@ The rest resolve when reached, not now.
   in canonical events (A10) and both renderers, storage that keeps the durable log
   (A2) small, and image tokens in the context budget. How well a model sees is
   measured when that work starts, not before.
+
 ---
 
 ## Repository Layout