feat: add us-cpa return model and calculations

skills/us-cpa/tests/test_returns.py

@@ -0,0 +1,48 @@
+from __future__ import annotations
+
+import unittest
+
+from us_cpa.returns import normalize_case_facts, resolve_required_forms, tax_on_ordinary_income
+
+
+class ReturnModelTests(unittest.TestCase):
+    def test_normalize_case_facts_computes_basic_1040_totals(self) -> None:
+        normalized = normalize_case_facts(
+            {
+                "taxpayer.fullName": "Jane Doe",
+                "filingStatus": "single",
+                "wages": 50000,
+                "taxableInterest": 100,
+                "federalWithholding": 6000,
+            },
+            2025,
+        )
+
+        self.assertEqual(normalized["requiredForms"], ["f1040"])
+        self.assertEqual(normalized["deductions"]["standardDeduction"], 15750.0)
+        self.assertEqual(normalized["totals"]["adjustedGrossIncome"], 50100.0)
+        self.assertEqual(normalized["totals"]["taxableIncome"], 34350.0)
+        self.assertEqual(normalized["totals"]["refund"], 2116.5)
+
+    def test_resolve_required_forms_adds_business_and_interest_forms(self) -> None:
+        normalized = normalize_case_facts(
+            {
+                "filingStatus": "single",
+                "wages": 0,
+                "taxableInterest": 2000,
+                "businessIncome": 12000,
+            },
+            2025,
+        )
+
+        self.assertEqual(
+            resolve_required_forms(normalized),
+            ["f1040", "f1040sb", "f1040sc", "f1040se", "f1040s1"],
+        )
+
+    def test_tax_bracket_calculation_uses_2025_single_rates(self) -> None:
+        self.assertEqual(tax_on_ordinary_income(34350.0, "single"), 3883.5)
+
+
+if __name__ == "__main__":
+    unittest.main()