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.
