# 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.