# Implementation Plan

## Checklist

1. Capture a parity baseline before product-code edits.
2. Add `blender/osmassets/building.py` with the current building assembly and
   detail helper logic.
3. Import `building.py` in `blender/generate_scene.py`.
4. Remove inline `_assemble_building()` and `add_building_details()` from
   `generate_scene.py`.
5. Replace the building call site with `building.assemble(...)`, passing
   `args["office_overrides"]` instead of the full `args` dict.
6. Run Python validation:
   - `python3 -m py_compile blender/osmassets/building.py`
   - `python3 -m py_compile blender/generate_scene.py`
   - `python3 -m unittest blender/tests/test_pure.py`
7. Capture parity after the refactor.
8. Compare before/after parity snapshots and inspect any non-ignored diff.
9. Update `.trellis/spec` and `docs/changelog.md` only for durable status or
   convention changes discovered during implementation.
10. Commit, archive the task, and record the journal entry.

## Validation Commands

```bash
node scripts/parity.js capture building-module-before --stages blender,cesium
python3 -m py_compile blender/osmassets/building.py
python3 -m py_compile blender/generate_scene.py
python3 -m unittest blender/tests/test_pure.py
node scripts/parity.js capture building-module-after --stages blender,cesium
node scripts/parity.js compare building-module-before building-module-after
```

## Risk Points

- Passing `office_overrides` instead of `args` changes the function contract;
  verify `office_override` custom properties and industrial classification via
  parity.
- `Building_<way_id>_Windows` mesh construction depends on `MeshBatch` object
  naming and `add_wall_panel()` geometry; copy it mechanically.
- `make_prism()` can return `None` on degenerate input, but the current call
  site filters `len(ring) >= 3`. Preserve the existing behavior and return
  tuple shape.
- Do not reorder building material creation or catalog entries; material order
  affects GLB material indices.
- Parity is mandatory because this is a pure refactor of bpy-layer scene
  generation.