Refactor fountain assembly module

.trellis/tasks/08-03-extract-fountain-module/implement.md

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+# 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.