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/03_context_and_scope.adoc

@@ -0,0 +1,30 @@
+ifndef::imagesdir[:imagesdir: ./images]
+
+[[section-context-and-scope]]
+== Context and Scope
+
+=== Business Context
+
+The system is a self-contained single-player game. It has no server backend, no multiplayer, and no live service dependencies. Its communication partners are:
+
+[cols="1,2,2" options="header"]
+|===
+| Partner | Inbound | Outbound
+| Player (keyboard/mouse) | WASD, mouse, R (reset), Escape (pause), Space (jump) | Rendered 3D scene + HUD
+| OSM data | Extracted building footprints, street geometry, elevation | Attribution string at runtime
+| Filesystem | Engine content (`.uasset`, `.umap`), Blender sources (`.blend`), DEM GeoTIFF | Saved maps, screenshots, packaged build
+|===
+
+=== Technical Context
+
+[cols="1,2,2" options="header"]
+|===
+| Channel | Protocol / Format | Purpose
+| Keyboard + mouse | Enhanced Input (IA_*, IMC_Default) | First-person movement, look, jump, reset, pause
+| Vulkan RHI | GPU rendering | Scene display at 60 FPS / 1440p
+| FBX | Autodesk FBX (binary) | Blender→Unreal mesh and skeleton transfer
+| GeoTIFF | 3DEP 1 m DEM | Terrain heightfield source
+| OSM XML | Overpass API extract | Building footprint and street layout source
+| Git / LFS | HTTPS to Gitea remote | Source control, binary asset storage
+|===
+