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

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

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+# Design
+
+## Architecture
+
+`fountain.py` will join the existing bpy-layer feature modules under
+`blender/osmassets/`. It may import `bpy`, `math`, and the collection helper
+needed to preserve current object linking behavior.
+
+The module will expose one function:
+
+```python
+def assemble(name, x, y, collection, materials):
+    ...
+```
+
+The signature mirrors the existing inline `add_fountain(name, x, y, collection,
+materials)` call site to keep the migration mechanical and parity-friendly.
+
+## Boundaries
+
+- `generate_scene.py` keeps OSM feature filtering, projection, material creation,
+  and count ownership.
+- `fountain.py` owns only Blender object creation for one fountain at already
+  projected `(x, y)` coordinates.
+- No pure-Python module may import `bpy`; this module is explicitly in the
+  bpy layer, like `water.py`, `grass.py`, `scrub.py`, and `tree.py`.
+
+## Data Flow
+
+1. `generate_scene.py` creates `fountain_mats` from `catalog.MATERIALS`.
+2. `generate_scene.py` finds point features where `tags.amenity == "fountain"`.
+3. `Projector.xy()` converts the OSM coordinate to local meters.
+4. `fountain.assemble("Fountain_" + id, fx, fy, props_c, fountain_mats)` creates
+   the existing basin, water disk, pedestal, crown, and droplets.
+5. `generate_scene.py` increments `counts["fountain_count"]`.
+
+## Compatibility
+
+The refactor must preserve all generated object names, primitive dimensions,
+materials, smoothing flags, and custom properties. Cesium export reads the
+resulting `.blend`; it should see an equivalent scene graph.
+
+## Trade-Offs
+
+A full feature registry is intentionally deferred. Starting with a one-module
+extraction keeps the diff small and lets parity isolate any regression to the
+fountain move before higher-risk building or road work.
+
+## Rollback
+
+Rollback is straightforward: remove `fountain.py`, restore the inline
+`add_fountain()` function, remove the module import, and restore the original
+call site.