Register arc42-v1 schema for single-file architecture documents

Twelve required chapters, checklist recommended. First-wave estate
documents validate against it.

markitect/schemas/schema-catalog.yaml

@@ -48,6 +48,24 @@ schemas:
     created: "2026-01-04"
     updated: "2026-01-04"
 
+  - id: "arc42-v1"
+    name: "arc42 Architecture Documentation Schema"
+    file: "arc42-schema-v1.0.md"
+    version: "1.0"
+    description: "Schema for a single-file arc42 architecture document (chapters 1–12 required)"
+    type: "document-schema"
+    usage: "Validates first-wave and complete arc42 system documents"
+    document_types:
+      - architecture
+      - arc42
+    tags:
+      - documentation
+      - architecture
+      - arc42
+    author: "MarkiTect Project"
+    created: "2026-08-19"
+    updated: "2026-08-19"
+
 # Future schemas to add:
 #
 # - id: "manpage-v1"
@@ -58,10 +76,6 @@ schemas:
 #   name: "API Reference Schema"
 #   description: "Schema for API endpoint documentation"
 #
-# - id: "arc42-v1"
-#   name: "arc42 Architecture Documentation Schema"
-#   description: "Schema for arc42 architecture documentation template"
-#
 # - id: "adr-v1"
 #   name: "Architecture Decision Record Schema"
 #   description: "Schema for ADR (Architecture Decision Record) documents"