docs: mark Step 3.5 as complete in PLAN.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

PLAN.md

@@ -125,11 +125,11 @@ Check off items as completed. Each phase builds on the previous.
 - [x] **Commit checkpoint** (b89ea41)
 
 ### Step 3.5: Animal Attributes Update
-- [ ] Update projections for AnimalAttributesUpdated event
-- [ ] Close old attribute intervals, open new ones for changed attrs
-- [ ] Add update_attributes to services/animal.py
-- [ ] Write tests: only changed attrs create new intervals
-- [ ] **Commit checkpoint**
+- [x] Update projections for AnimalAttributesUpdated event
+- [x] Close old attribute intervals, open new ones for changed attrs
+- [x] Add update_attributes to services/animal.py
+- [x] Write tests: only changed attrs create new intervals
+- [x] **Commit checkpoint** (dc7700d)
 
 ### Step 3.6: Animal Tagging
 - [ ] Create migration for tag_suggestions table