feat: add product collection event handling

Implements Step 4.3 from the plan:
- Add selection/resolver.py with basic resolve_selection for validating
  animal IDs exist and are alive
- Add ProductsProjection placeholder (stats tables added in Step 4.4)
- Add ProductService with collect_product() function
- Add PRODUCT_COLLECTED to EventAnimalsProjection for linking events
  to affected animals
- Full test coverage for all new components

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

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

tests/test_selection_resolver.py

@@ -0,0 +1,139 @@
+# ABOUTME: Tests for basic selection resolver.
+# ABOUTME: Tests animal ID validation for product collection.
+
+import time
+
+import pytest
+
+from animaltrack.events.payloads import AnimalCohortCreatedPayload
+from animaltrack.events.store import EventStore
+from animaltrack.projections import ProjectionRegistry
+from animaltrack.projections.animal_registry import AnimalRegistryProjection
+from animaltrack.projections.event_animals import EventAnimalsProjection
+from animaltrack.projections.intervals import IntervalProjection
+from animaltrack.selection import SelectionResolverError, resolve_selection
+from animaltrack.services.animal import AnimalService
+
+
+@pytest.fixture
+def event_store(seeded_db):
+    """Create an EventStore for testing."""
+    return EventStore(seeded_db)
+
+
+@pytest.fixture
+def projection_registry(seeded_db):
+    """Create a ProjectionRegistry with animal projections registered."""
+    registry = ProjectionRegistry()
+    registry.register(AnimalRegistryProjection(seeded_db))
+    registry.register(EventAnimalsProjection(seeded_db))
+    registry.register(IntervalProjection(seeded_db))
+    return registry
+
+
+@pytest.fixture
+def animal_service(seeded_db, event_store, projection_registry):
+    """Create an AnimalService for testing."""
+    return AnimalService(seeded_db, event_store, projection_registry)
+
+
+@pytest.fixture
+def valid_location_id(seeded_db):
+    """Get a valid location ID from seeds."""
+    row = seeded_db.execute("SELECT id FROM locations WHERE name = 'Strip 1'").fetchone()
+    return row[0]
+
+
+def make_cohort_payload(
+    location_id: str,
+    count: int = 3,
+    species: str = "duck",
+) -> AnimalCohortCreatedPayload:
+    """Create a cohort payload for testing."""
+    return AnimalCohortCreatedPayload(
+        species=species,
+        count=count,
+        life_stage="adult",
+        sex="unknown",
+        location_id=location_id,
+        origin="purchased",
+    )
+
+
+@pytest.fixture
+def animal_ids(seeded_db, animal_service, valid_location_id):
+    """Create a cohort and return the animal IDs."""
+    payload = make_cohort_payload(valid_location_id, count=5)
+    ts_utc = int(time.time() * 1000)
+    event = animal_service.create_cohort(payload, ts_utc, "test_user")
+    return event.entity_refs["animal_ids"]
+
+
+class TestResolveSelectionValid:
+    """Tests for resolve_selection with valid inputs."""
+
+    def test_returns_validated_ids_when_all_exist(self, seeded_db, animal_ids):
+        """resolve_selection returns the IDs when all are valid and alive."""
+        result = resolve_selection(seeded_db, animal_ids)
+
+        assert result == animal_ids
+
+    def test_handles_single_animal(self, seeded_db, animal_ids):
+        """resolve_selection works with a single animal."""
+        single_id = [animal_ids[0]]
+
+        result = resolve_selection(seeded_db, single_id)
+
+        assert result == single_id
+
+    def test_handles_subset_of_animals(self, seeded_db, animal_ids):
+        """resolve_selection works with a subset of animals."""
+        subset = animal_ids[:2]
+
+        result = resolve_selection(seeded_db, subset)
+
+        assert result == subset
+
+
+class TestResolveSelectionErrors:
+    """Tests for resolve_selection error cases."""
+
+    def test_raises_for_nonexistent_animal(self, seeded_db, animal_ids):
+        """Raises SelectionResolverError for animal not found."""
+        fake_id = "01ARZ3NDEKTSV4RRFFQ69G5FAV"
+        ids_with_fake = animal_ids[:1] + [fake_id]
+
+        with pytest.raises(SelectionResolverError, match="not found"):
+            resolve_selection(seeded_db, ids_with_fake)
+
+    def test_raises_for_dead_animal(self, seeded_db, animal_ids):
+        """Raises SelectionResolverError for animal with status != 'alive'."""
+        # Mark the first animal as dead
+        dead_id = animal_ids[0]
+        seeded_db.execute(
+            "UPDATE animal_registry SET status = 'dead' WHERE animal_id = ?",
+            (dead_id,),
+        )
+
+        with pytest.raises(SelectionResolverError, match="not alive"):
+            resolve_selection(seeded_db, [dead_id])
+
+    def test_raises_for_mixed_valid_invalid(self, seeded_db, animal_ids):
+        """Raises SelectionResolverError when mix of valid and invalid animals."""
+        # Mark one as dead
+        dead_id = animal_ids[0]
+        seeded_db.execute(
+            "UPDATE animal_registry SET status = 'dead' WHERE animal_id = ?",
+            (dead_id,),
+        )
+
+        # Mix: one dead, one alive
+        mixed_ids = [dead_id, animal_ids[1]]
+
+        with pytest.raises(SelectionResolverError):
+            resolve_selection(seeded_db, mixed_ids)
+
+    def test_raises_for_empty_list(self, seeded_db):
+        """Raises SelectionResolverError for empty resolved_ids list."""
+        with pytest.raises(SelectionResolverError, match="empty"):
+            resolve_selection(seeded_db, [])