M2.4.4 End-to-end test for snow insulation and frozen-soil effects

**Summary**

Verify integrated behavior of snow insulation, frozen-soil limitation, and hydrologic/ecological consequences.

**Purpose**

Ensure Milestone 2 behaves qualitatively as intended and remains backward-compatible with warm climates.

**Tasks**

- [ ] Create synthetic forcing scenarios that produce:
- persistent winter snow cover
- intermittent freezing around 0C
- a warm control climate with no snow or freezing
- [ ] Evaluate time series of soil temperature, frozen-soil fraction, infiltration, runoff, plant uptake limitation, and bare-soil evaporation suppression.
- [ ] Confirm expected qualitative seasonal behavior.
- [ ] Check backward compatibility for warm climates.

**Acceptance criteria**

- [ ] Snow-covered winter soils are less cold than air in appropriate test cases.
- [ ] Frozen periods reduce infiltration and plant uptake.
- [ ] Warm control climates show no spurious cold-region effects.
- [ ] Diagnostic plots and summary notes are saved.

**Suggested tests**

- [ ] Automated regression plots for all synthetic climates.

Start by locating the existing end-to-end test infrastructure and synthetic forcing setup in the repository. Create scenarios for persistent snow, intermittent freezing, and warm control conditions, then evaluate the listed soil, hydrologic, and ecological time series. Done means the qualitative acceptance criteria are verified and diagnostic plots and summary notes are saved.

Written by the indexing model from the issue text.