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

.trellis/tasks/archive/2026-08/08-03-extract-fountain-module/prd.md

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+# Extract fountain module
+
+## Goal
+
+Move the fountain assembly code out of `blender/generate_scene.py` into a
+dedicated bpy-layer module, keeping the generated `.blend` / `.glb` structure
+unchanged. This is the first low-risk step toward the documented P2 feature
+module / registry direction.
+
+## Background
+
+- `.trellis/spec/guides/artifact-parity-guide.md` records P2 as partial:
+  `water.py`, `grass.py`, `scrub.py`, and `tree.py` exist, but `building`,
+  `fountain`, and `roads` still live in `generate_scene.py`.
+- `blender/generate_scene.py:639` defines `add_fountain()` inline.
+- `blender/generate_scene.py:895` dispatches OSM point features with
+  `amenity=fountain`, calls `add_fountain("Fountain_" + id, ...)`, and
+  increments `counts["fountain_count"]`.
+- `blender/osmassets/catalog.py` already owns the three fountain materials:
+  `fountain_stone`, `fountain_water`, and `fountain_spray`.
+- This task is a pure refactor. It must not change fountain geometry, object
+  names, materials, custom properties, counts, stdout markers, or Cesium export
+  behavior.
+
+## Requirements
+
+1. Add a new bpy-layer module at `blender/osmassets/fountain.py`.
+2. Move the current fountain assembly behavior from `generate_scene.py` into
+   the new module.
+3. Keep `generate_scene.py` responsible for:
+   - creating fountain materials from `catalog.MATERIALS`;
+   - filtering `point_features` for `amenity=fountain`;
+   - projecting coordinates;
+   - incrementing `counts["fountain_count"]`.
+4. Preserve the existing object names:
+   - `Fountain_<id>_Basin`
+   - `Fountain_<id>_Water`
+   - `Fountain_<id>_Pedestal`
+   - `Fountain_<id>_Water_Crown`
+   - `Fountain_<id>_Droplet_<n>`
+5. Preserve the existing basin custom property:
+   `osm_feature = "amenity=fountain"`.
+6. Do not introduce a full `features/` registry in this task.
+7. Do not modify `building`, `roads`, material definitions, `MATERIALS` order,
+   `ROAD_LAYERS` order, stdout markers, or parity ignore lists.
+8. Do not fix unrelated documented defects D1, D2, or D3.
+
+## Acceptance Criteria
+
+- [x] `blender/osmassets/fountain.py` contains the fountain assembly behavior
+      and imports only bpy-layer-safe dependencies.
+- [x] `blender/generate_scene.py` imports and calls the new fountain module;
+      no inline `add_fountain()` function remains there.
+- [x] Scene counts and `SCENE_DONE` JSON keys remain unchanged.
+- [x] `python3 -m unittest blender/tests/test_pure.py` passes.
+- [x] `python3 -m py_compile blender/osmassets/fountain.py` passes.
+- [x] `python3 -m py_compile blender/generate_scene.py` passes.
+- [x] A before/after parity comparison is run for the Blender/Cesium stages and
+      any non-ignored diff is either absent or explicitly explained as
+      expected. For this refactor, the expected result is no contract diff.
+
+## Out Of Scope
+
+- Full feature registry design or implementation.
+- Building extraction.
+- Road extraction.
+- Fountain visual redesign or geometry changes.
+- Material contract changes.
+- Documentation-only cleanup of stale comments such as D3.
+
+## Open Questions
+
+None.