chore(task): archive 08-03-extract-building-module

.trellis/tasks/archive/2026-08/08-03-extract-building-module/design.md

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+# Design
+
+## Architecture
+
+`building.py` will join the bpy-layer feature modules under
+`blender/osmassets/`. It may import `MeshBatch`, `add_roof`,
+`add_wall_panel`, and `make_prism` from `osmassets.mesh`, plus
+`parse_height` from `osmassets.osm`.
+
+The module will expose:
+
+```python
+def assemble(ring, way_id, tag, office_overrides, collection, materials):
+    ...
+```
+
+The function returns:
+
+```python
+(added, industrial_added, ring_pts)
+```
+
+This keeps `generate_scene.py`'s count and focus-point handling unchanged while
+removing building geometry construction from the entry script.
+
+## Boundaries
+
+- `generate_scene.py` keeps CLI parsing, OSM way dispatch, material creation,
+  count accumulation, focus-point accumulation, scene custom properties, and
+  stdout markers.
+- `building.py` owns only building object creation and building-specific facade
+  details for a ring that has already been projected.
+- `building.py` is a bpy-layer module. It must not be imported by pure Python
+  modules or pure unit tests.
+- `catalog.py` remains the material declaration source; no material entries or
+  order change in this task.
+
+## Data Flow
+
+1. `generate_scene.py` parses `args["office_overrides"]` into a set.
+2. `generate_scene.py` creates `building_mats` from `catalog.MATERIALS`.
+3. OSM ways are projected to local-meter rings in `generate_scene.py`.
+4. For ways with a `building` tag and at least three ring points,
+   `building.assemble(ring, str(way["id"]), tag, args["office_overrides"],
+   buildings_c, building_mats)` creates the prism, roof, windows, custom
+   properties, and bevel.
+5. `generate_scene.py` updates `counts["building_count"]`,
+   `counts["industrial_count"]`, and `focus_points` from the return tuple.
+
+## Compatibility
+
+The move must preserve object names, mesh names, material assignments, bevel
+modifier fields, custom properties, window-band geometry, and return values.
+Cesium export consumes the resulting `.blend`, so parity should see no
+non-ignored contract difference.
+
+## Trade-Offs
+
+The `office_overrides` set is passed into `building.assemble()` rather than
+passing the whole `args` dict. This narrows the module contract while preserving
+behavior. A full feature registry remains deferred until building and roads are
+both isolated enough to compare safely.
+
+## Rollback
+
+Rollback is mechanical: move `assemble()` and its helper back into
+`generate_scene.py`, restore direct calls, remove the `building.py` import, and
+delete the module.