docs: arc42 architecture documentation for Gig Line

Populated from the arc42-template (EN/adoc) with project-specific content
across all 12 sections:

1.  Introduction and Goals — requirements overview, quality goals, stakeholders
2.  Architecture Constraints — technical (engine, collision, coordinates),
    organisational (squash-merge, visual review, provenance), conventions
3.  Context and Scope — business and technical context tables
4.  Solution Strategy — technology decisions, top-level decomposition,
    quality achievement approach
5.  Building Block View — Gameplay (6 C++ classes), Site (7 meshes + 7
    materials), Facades (9 buildings), Art Pipeline, QA & Review; Level 2
    white boxes for input flow and collision model
6.  Runtime View — walk, screenshot capture, QA self-test, build/import
    pipeline scenarios
7.  Deployment View — dev workstation and packaged build infrastructure
8.  Cross-cutting Concepts — coordinate system, collision model, lighting,
    materials, data provenance, QA harness
9.  Architecture Decisions — 7 ADRs (dynamic lighting, UE materials,
    C++ collision helper, grounded reset, core ticker, Y convention,
    headless Blender)
10. Quality Requirements — overview table + 7 measurable scenarios
11. Risks and Technical Debts — 5 risks, 2 debts with mitigations
12. Glossary — 16 domain-specific terms

Master document: Docs/Architecture/architecture.adoc
Config: Docs/Architecture/config.adoc

Docs/Architecture/about-arc42.adoc

@@ -0,0 +1,16 @@
+:homepage: https://arc42.org
+
+:keywords: software-architecture, documentation, arc42, gig-line
+
+:numbered!:
+**About arc42**
+
+[role="lead"]
+arc42, the template for documentation of software and system architecture.
+
+Template Version {revnumber}. {revremark}, {revdate}
+
+Created, maintained and (C) by Dr. Peter Hruschka, Dr. Gernot Starke and contributors.
+See https://arc42.org.
+
+Adapted for the {project-name} project ({project-desc}).