docs: convert all architecture .adoc sections to markdown, prettier-format all .md

- Converted 13 individual .adoc section files to .md via
  asciidoctor→docbook5→pandoc(gfm) + post-processing, placed alongside
  the .adoc sources in Docs/Architecture/
- Rebuilt architecture.md from the individual section .md files
- Ran prettier --write on all .md in the repo (23 files changed):
  normalised table column alignment, trailing whitespace, line breaks
- Added .agents/ to .prettierignore (vendored third-party content)
- All .md now passes prettier --check

Docs/Architecture/08_concepts.md

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+# Cross-cutting Concepts
+
+## Coordinate system and scale
+
+- **Source CRS**: EPSG:26911 (UTM zone 11N). Local origin recorded in `Geo/`.
+
+- **Blender**: metres. East → +X, north → +Y (Blender convention).
+
+- **Unreal**: centimetres. East → +X, north → **−Y** (confirmed from street mesh: `Sidewalk_North` Y −1912..−1053, `Sidewalk_South` Y +718..+1447).
+
+- **Conversion**: every exporter calls the metre→centimetre function. Import presets (`ue_metric_v1`, `ue_character_v1`) are named and repeatable.
+
+Rule: place a building only against a neighbour or street mesh, never in isolation.
+
+## Collision model
+
+- Site meshes (road, sidewalks, kerbs, crossing): `STATIC` mobility, `BlockAll` collision profile.
+
+- Terrain: `STATIC`, `CTF_UseComplexAsSimple` (forced by `UGigLineAssetTools` at build time, saved on the asset — the flag lives on the `UStaticMesh`/`UBodySetup`, not the map).
+
+- Buildings: `STATIC`, simple collision (convex hull from mesh).
+
+- Rule: the collision flag must be saved on the asset. Saving only the `.umap` silently reverts it.
+
+## Lighting
+
+- Dynamic-only: `r.AllowStaticLighting=False`. All lights are `MOVABLE`.
+
+- Fixed daytime rig: directional light (sun, −62° pitch), sky light, exponential height fog, post-process volume with fixed exposure.
+
+- Rationale: avoids "LIGHTING NEEDS TO BE REBUILT" warnings; no bake step; the scene never changes time of day.
+
+## Materials
+
+- All materials authored in Unreal, not carried from Blender (FBX import loses diffuse colours).
+
+- World-space-noise blend nodes for cheap procedural variation (asphalt grain, concrete speckle, stucco roughness).
+
+- Instance parameters where tunable (e.g. wetness, ageing).
+
+## Geographic data provenance
+
+- OSM extract is layout data only — not authoritative for façade, height, or pavement detail.
+
+- DEM from 3DEP 1 m; elevation verified against cross-sections in `Geo/verification/`.
+
+- Reference photography (local, gitignored) drives manual façade work.
+
+- In-game ODbL attribution drawn by `AGigLineHUD` every frame, bottom-left.
+
+## QA harness
+
+- `-qaselftest`: core ticker (not world timer) so pause cannot freeze the test that proves pause works.
+
+- `GigLine SIM` / `GigLine GROUND`: TEMP diagnostic logging, to be removed before release tag.
+
+- Screenshot approval: human must tick `Screenshots/README.md` before visual work is marked done.