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/02_architecture_constraints.adoc

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+ifndef::imagesdir[:imagesdir: ./images]
+
+[[section-architecture-constraints]]
+== Architecture Constraints
+
+=== Technical Constraints
+
+[cols="1,3" options="header"]
+|===
+| Constraint | Explanation
+| Engine: Unreal 5.8.4 | Pinned; no bundled Linux clang — uses system clang 23.1.4. Game targets use `BuildSettingsVersion.V7`.
+| Platform: Linux first | Developed and tested on Bazzite Linux (Fedora 45 base). Windows builds ship but are secondary.
+| GPU: RTX 5070 Ti 16 GB | Dev workstation target. Vulkan RHI; open kernel module driver 615.78.08.
+| No static lighting | `r.AllowStaticLighting=False`. The demo is a fixed daytime scene lit entirely dynamically to avoid "LIGHTING NEEDS TO BE REBUILT" warnings.
+| Blender→UE via FBX | Flat diffuse colours do not survive FBX→Unreal import. All materials are authored in UE.
+| Terrain collision | UE 5.8 exposes `CollisionTraceFlag` (not `bUseComplexAsSimpleCollision`); Python cannot set it. Must use C++ helper `UGigLineAssetTools::SetComplexCollisionAsSimple`.
+| Coordinate convention | South = +Y, north = −Y (ground truth = street mesh orientation). 1 m in Blender = 100 cm in Unreal.
+|===
+
+=== Organisational Constraints
+
+[cols="1,3" options="header"]
+|===
+| Constraint | Explanation
+| Squash-merge to `main` | Every PR is squash-merged for a linear history. No direct commits to `main` except integration merges.
+| Visual review required | Any change affecting appearance must produce screenshots for human approval before being marked done.
+| Asset provenance | Every committed asset must have verified licence and redistribution permission recorded in `THIRD_PARTY_NOTICES.md`.
+| Reference photography | Local only, gitignored, never committed. Used for façade work under fair-use / internal-development rationale.
+| LFS for binaries | `.uasset`, `.umap`, `.blend` tracked with Git LFS; `Binaries/`, `Intermediate/`, `Saved/` are gitignored.
+|===
+
+=== Conventions
+
+[cols="1,3" options="header"]
+|===
+| Convention | Explanation
+| Semantic versioning | `MAJOR.MINOR.PATCH`. `0.x` documents breaking changes. `1.0.0` = agreed game scope.
+| Task tracking in `TODO.md` | Single central task list. `- [ ]` open, `- [-]` or `- [~]` in progress, `- [x]` done, `- [A]` active, `- [D]` deferred.
+| Headless Blender | All Blender work driven by `bpy` scripts in `Tools/`, not MCP. Blender skills are method guides only.
+| Named export presets | `ue_metric_v1` for static meshes, `ue_character_v1` for skeletal. Every exporter calls the metre→centimetre conversion.
+|===
+