docs(docs): add frontmatter to architecture docs

Standardized YAML frontmatter on all 10 docs/architecture/ files with
title, description, type, status, ticket, decision_refs, and author
fields. Enables context-aware document loading.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/architecture/z-layer-gap-analysis.md

@@ -1,3 +1,15 @@
+---
+title: "Z-Layer Rendering Pipeline — Gap Analysis"
+description: "Gap analysis for multi-floor z-layer rendering — y-sort occlusion contract, airborne rendering, lower floor compositing, fog interaction, and performance budgets"
+type: architecture
+status: active
+ticket: ""
+decision_refs: [D-049]
+author: "Tyre"
+created: YYYY-MM-DD
+updated: YYYY-MM-DD
+---
+
 # Z-Layer Rendering Pipeline — Gap Analysis
 
 Author: Tyre (architecture) | Sprint: 6 | Decision: D-049 amendment