fix(dupont): reject out-of-domain window + bidirectional clip + suffix discipline (R1-R4/M2)

Per finance ruling 2026-10-05 (project-20261005-dupont-oob-window-fix):

- R1: no intersection with available month domain => explicit error (endpoint 400), no window fields

- R2: with intersection => intersection window, months>=1 (kills inversion and months<=0)

- R3: quarter/half/... suffix kept only when returned window == nominal window (clipped flag)

- R4: symmetric lower-bound clip note; M2: TTM nominal window = [anchor-11, anchor] (12 months)

- M1: docstring correction (lower-bound overflow is clipped, not 400)

No push / no deploy / no restart / no DB write.

backend/tests/test_dupont_period_options.py

@@ -100,10 +100,14 @@ def test_ytd_ttm_unchanged():
     assert (ytd["period_start"], ytd["period_end"]) == ("2026-01", "2026-10")
     assert ytd["months"] == 10
 
+    # M2（2026-10-05 裁定 §4）：TTM 名义区间 = [anchor−11, anchor] = **12 个月**。
+    # 历史断言写的是 13 个月（anchor 的「去年同月」），即被裁定为缺陷的口径，见
+    # test_dupont_oob_window_ruling_20261005.py / 本文件 §裁定回归。
     ttm = rw("ttm", anchor="2026-10")
     assert ttm["period_key"] == "ttm"
-    assert (ttm["period_start"], ttm["period_end"]) == ("2025-10", "2026-10")
-    assert ttm["months"] == 13
+    assert (ttm["period_start"], ttm["period_end"]) == ("2025-11", "2026-10")
+    assert ttm["nominal_months"] == 12
+    assert ttm["months"] == 12
 
 
 def test_custom_unchanged():