chore(task): archive 08-03-design-feature-registry

.trellis/tasks/archive/2026-08/08-03-design-feature-registry/implement.md

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+# Implementation Plan
+
+## Checklist
+
+1. Capture a parity baseline before product-code edits:
+   `node scripts/parity.js capture feature-registry-before --stages blender,cesium`
+2. Add `blender/osmassets/features.py` with a small `FeatureHandler` contract
+   and `dispatch_ways(...)`.
+3. Import `features.py` in `blender/generate_scene.py`.
+4. Convert the current OSM way `if` / `elif` chain into ordered local handlers:
+   water, grass, scrub, tree-row, building.
+5. Keep handler bodies mechanically equivalent to the current branch bodies.
+6. Leave road GeoJSON/fallback, tree placement, fountain point processing,
+   scene metadata, and `SCENE_DONE` behavior unchanged except for any small
+   phase comments needed to clarify ordering.
+7. Run Python checks:
+   - `python3 -m py_compile blender/osmassets/features.py`
+   - `python3 -m py_compile blender/generate_scene.py`
+   - `python3 -m unittest blender/tests/test_pure.py`
+8. Capture parity after the refactor:
+   `node scripts/parity.js capture feature-registry-after --stages blender,cesium`
+9. Compare parity:
+   `node scripts/parity.js compare feature-registry-before feature-registry-after`
+10. Update `.trellis/spec` and `docs/changelog.md` only for durable registry
+    conventions or P2 status changes discovered during implementation.
+11. Commit, archive the task, and record the journal entry.
+
+## Validation Commands
+
+```bash
+node scripts/parity.js capture feature-registry-before --stages blender,cesium
+python3 -m py_compile blender/osmassets/features.py
+python3 -m py_compile blender/generate_scene.py
+python3 -m unittest blender/tests/test_pure.py
+node scripts/parity.js capture feature-registry-after --stages blender,cesium
+node scripts/parity.js compare feature-registry-before feature-registry-after
+```
+
+## Risk Points
+
+- Handler order is equivalent to the old `elif` chain. Changing it can alter
+  which feature claims a way.
+- Object creation phase order affects `.blend` and GLB digests.
+- Material creation order affects GLB material indices.
+- Tree rows and scrub interior trees feed the later tree placement phase; do
+  not place trees inside the way dispatch loop.
+- `catalog.check_layers()` must stay warning-only and keep the exact printed
+  prefix.
+- Do not add a new road layer id list; keep deriving road layers from
+  `catalog.ROAD_LAYERS`.
+
+## Rollback Point
+
+If parity shows a non-ignored difference, first compare object order, object
+names, mesh/material names, `SCENE_DONE`, and scene custom properties. If the
+cause is not obvious, revert the registry dispatch and return to the current
+way loop before trying a narrower refactor.