M1.2.2 Improve daily interpolation of monthly temperatures if needed

**Summary**
Implement a smoother daily temperature interpolation scheme if the current method creates artifacts that affect snow partition or melt timing. basically how can we translate the subdaily info in a monthly model timestep?

**Purpose**
Reduce unrealistic month-boundary jumps and improve daily-scale freeze–thaw realism while preserving the monthly climate signal.

**Tasks**
- [ ] Interpolate monthly means smoothly to daily values.
- [ ] Optionally add small deterministic short-period variability to permit realistic daily threshold crossings.
- [ ] Preserve target monthly means within a documented tolerance.
- [ ] Handle year boundaries smoothly.
- [ ] Document the chosen method.

**Acceptance criteria**

- [ ] Daily temperature series is smooth across month boundaries.
- [ ] Reconstructed monthly means match target monthly means within tolerance.
- [ ] The new method is documented and available to the snow module.

**Suggested tests**

- [ ] Synthetic annual cycle with strong seasonality.
- [ ] Constant-temperature annual test.
- [ ] Comparison plots of old versus new interpolation.

Start by locating the snow module's current monthly-to-daily temperature interpolation. Use the suggested synthetic annual-cycle and constant-temperature tests, then compare old and new interpolation plots. Done means smooth month boundaries, monthly means within a documented tolerance, smooth year boundaries, and documentation of the chosen method.

Written by the indexing model from the issue text.