feat: Safety Manager + Traceability + PlantUML in CI

- Implement Safety Manager (SWA-001, ASIL-D): Hill-Hold + Auto-Apply
  state machine, 13 unit tests
- Update SWA-002 + SWA-001 link coverage so all SWE reqs are covered
- New tool: tools/traceability.py — Markdown-frontmatter-basierter
  Traceability-Checker + HTML/JSON-Matrix-Generator (Doorstop-Format ohne
  Doorstop-Dependency)
- New tool: tools/render_plantuml.py — extrahiert PlantUML-Bloecke aus
  arch/**.md und rendert via plantuml.com zu SVG
- validate.yml: neue Steps Traceability-Check, Matrix-Publish, PlantUML-
  Render; uploaded als Gitea-Artefakte

tools/generate_doorstop_items.py

@@ -439,7 +439,7 @@ SA_ELEMENTS = [
 SWA_ELEMENTS = [
     {
         "id": "SWA-001", "asil": "D",
-        "links": ["SWE-007", "SWE-008", "SWE-009", "SWE-010"],
+        "links": ["SWE-007", "SWE-008", "SWE-009", "SWE-010", "SWE-011", "SWE-012"],
         "title": "Safety Manager",
         "text": textwrap.dedent("""
         ## Verantwortung
@@ -500,7 +500,7 @@ SWA_ELEMENTS = [
     },
     {
         "id": "SWA-002", "asil": "D",
-        "links": ["SWE-001", "SWE-002", "SWE-003", "SWE-004"],
+        "links": ["SWE-001", "SWE-002", "SWE-003", "SWE-004", "SWE-005"],
         "title": "Apply Controller",
         "text": textwrap.dedent("""
         ## Verantwortung