feat: sync latest ARR implementation

tests/test_monthly_reports_xlsx.py

@@ -10,6 +10,8 @@ from datetime import date
 from decimal import Decimal
 from pathlib import Path
 
+from openpyxl import load_workbook
+
 from monthly_reports.contracts import (
     ChannelObservation,
     DailyVersionPin,
@@ -61,7 +63,7 @@ def report_payload() -> dict:
     report = build_monthly_report(
         2026,
         7,
-        date(2026, 7, 26),
+        date(2026, 7, 8),
         MonthlySnapshot(
             (source,),
             (DailyVersionPin(date(2026, 7, 8), 100),),
@@ -113,11 +115,24 @@ class MonthlyReportsXlsxTests(unittest.TestCase):
                 "LIANTAI-GROUP", "LIANTAI-FIT", "QBD", "DY-AI-Easy-KB", "FENGRUN"
             ])
             self.assertEqual(summary["row_counts"], [0, 0, 1, 0, 0])
-            self.assertEqual(summary["formula_count"], 0)
+            self.assertEqual(summary["formula_count"], 1)
             self.assertEqual(summary["preview_count"], 5)
             self.assertEqual(len(list(previews.glob("sheet-*.png"))), 5)
             self.assertEqual(output.stat().st_mode & 0o777, 0o600)
             self.assertFalse(Path(f"{output}.inspect.ndjson").exists())
+            workbook = load_workbook(output, data_only=False, read_only=True)
+            try:
+                self.assertEqual(workbook["QBD"]["S2"].value, "=R2*C2*G2")
+                formulas = [
+                    cell.value
+                    for sheet in workbook.worksheets
+                    for row in sheet.iter_rows()
+                    for cell in row
+                    if cell.data_type == "f"
+                ]
+                self.assertEqual(formulas, ["=R2*C2*G2"])
+            finally:
+                workbook.close()
 
     def test_invalid_payload_returns_only_stable_error_code(self):
         node = node_binary()