feat(portfolio): add stock consolidation domain and schema

tests/test_corporate_actions.py

@@ -0,0 +1,15 @@
+from decimal import Decimal
+from app.domain.corporate_actions import calculate_stock_consolidation
+
+def test_stock_consolidation_preserves_cost_and_scales_wac():
+    result = calculate_stock_consolidation(Decimal("1452"), Decimal("726"), Decimal("1086.23"))
+    assert result.average_price_after == Decimal("2172.46")
+    assert result.total_cost_basis == Decimal("1577205.96000000")
+    assert result.ratio == Decimal("0.5")
+
+def test_stock_consolidation_rejects_fractional_result():
+    import pytest
+    from app.domain.corporate_actions import CorporateActionError
+    with pytest.raises(CorporateActionError) as exc:
+        calculate_stock_consolidation(Decimal("3"), Decimal("1.5"), Decimal("10"))
+    assert exc.value.code == "FRACTIONAL_RESULT_UNSUPPORTED"