# Implementation Plan

## Checklist

1. Capture a parity baseline before product-code edits.
2. Add `blender/osmassets/fountain.py` with the current fountain assembly logic.
3. Import the module in `blender/generate_scene.py`.
4. Remove inline `add_fountain()` from `generate_scene.py`.
5. Replace the call site with `fountain.assemble(...)`.
6. Run Python validation:
   - `python3 -m py_compile blender/osmassets/fountain.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` or `docs/changelog.md` only if the work reveals a
   durable new constraint or user-visible change.
10. Commit, archive the task, and record the journal entry.

## Validation Commands

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

## Risk Points

- `link_object_to_collection()` must be used the same way as before so object
  collection membership stays unchanged.
- The basin custom property must remain on the basin object only.
- Droplet order and names must stay stable.
- `math` usage may move with the function; remove the import from
  `generate_scene.py` only if no remaining code uses it.
- Parity is mandatory because this is a pure refactor of bpy-layer scene
  generation.