docs: mark Step 3.3 as complete in PLAN.md

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PLAN.md

@@ -108,13 +108,13 @@ Check off items as completed. Each phase builds on the previous.
 - [x] **Commit checkpoint** (e3d6528)
 
 ### Step 3.3: Animal Cohort Creation
-- [ ] Create `projections/animal_registry.py` for cohort creation
-- [ ] Create `projections/intervals.py` for location and attribute intervals
-- [ ] Create `services/animal.py` with create_cohort function
-- [ ] Populate animal_registry, live_animals_by_location, event_animals
-- [ ] Create initial location and attribute intervals
-- [ ] Write tests: cohort validation, correct animal count, all projections updated
-- [ ] **Commit checkpoint**
+- [x] Create `projections/animal_registry.py` for cohort creation
+- [x] Create `projections/intervals.py` for location and attribute intervals
+- [x] Create `services/animal.py` with create_cohort function
+- [x] Populate animal_registry, live_animals_by_location, event_animals
+- [x] Create initial location and attribute intervals
+- [x] Write tests: cohort validation, correct animal count, all projections updated
+- [x] **Commit checkpoint** (876e817)
 
 ### Step 3.4: Animal Movement
 - [ ] Update projections for AnimalMoved event