M1.2.1 Review daily temperature generation for freeze-thaw realism

**Summary**
Assess whether the current daily temperature generator can support realistic snow accumulation and melt transitions.

**Purpose**
Snow accumulation, partial rain/snow partition, and melt timing depend strongly on daily temperature variability around 0C.

**Tasks**
- [ ] Inspect how monthly mean temperature and diurnal range could be converted into daily mean air temperature.
- [ ] Verify that the generated daily sequence can cross 0C in realistic shoulder-season months.
- [ ] Identify month-boundary artifacts that could create unrealistic snow onset or melt pulses.
- [ ] Evaluate whether the current forcing generator is adequate for snow partition and melt timing in at least one temperate and one continental synthetic climate.

**Acceptance criteria**
- [ ] Diagnostic plots exist for at least one temperate and one continental synthetic climate.
- [ ] The review states whether the current daily forcing generator is adequate as-is or needs modification.
- [ ] Any known limitations relevant to snow are documented.

**Suggested tests**
- [ ] Plot daily mean temperature for synthetic climates with winter months near freezing.
- [ ] Confirm plausible frequency of days above and below 0C.

Start by locating the current daily temperature generator and the tests or plotting entry points used for synthetic climates. Generate diagnostic plots for temperate and continental cases with winter temperatures near 0C, then inspect zero-crossing frequency and month-boundary transitions. Done means the review states whether the generator is adequate, records snow-related limitations, and includes the requested diagnostics.

Written by the indexing model from the issue text.