test: add synthetic model mode checks

[phiscu]

No description provided.

[phiscu]

Summary

This pull request expands the test coverage for MATILDA 1.0.2 before maintenance work begins.

It adds deterministic synthetic tests for:

• parameter initialization and validation;
• rain–snow separation and precipitation conservation;
• snow and ice melt, refreezing, and glacier-reservoir bookkeeping;
• simulations without glaciers;
• simulations with a fixed glacier area;
• leap-year handling, repeatability, and input-data preservation;
• one complete model evaluation through mspot.

Exact reference outputs are included for the zero-glacier and fixed-glacier configurations. These
references will make it possible to identify and quantify numerical changes caused by future
modifications to the core routines.

Scope

This pull request changes only tests, test documentation, and test configuration. It does not modify
the model implementation or its scientific behaviour.

The runoff data used for the mspot test are synthetic. They test data alignment, unit conversion, and
objective-function calculation, but they do not assess model performance or scientific validity.

Validation

• Full test suite: 22 passed
• Dependency consistency check: passed
• Wheel build and installed-wheel verification: passed
• Existing and new numerical reference comparisons: no changes detected

Remaining test gaps

Further tests are still needed for:

• complete glacier loss;
• positive cumulative glacier mass balance;
• serial-versus-parallel calibration equivalence.