test: add model output consistency checks

[phiscu]

Summary

This pull request establishes reproducible model output checks for MATILDA 1.0.2.

It:

• adds a pinned Python 3.11 test environment;
• compares the maintained 2000–2020 glacier-evolution scenario with the existing numerical reference;
• verifies the public output structure, numerical tables, KGE, summary statistics, and glacier-
  evolution outputs;
• produces variable-level and annual reports that quantify the effect of future model changes;
• checks package version metadata, parameter defaults, public imports, and wheel contents;
• runs the checks in GitHub Actions.

Purpose

The tests record the current model behavior before changes are made to the core routines. The reports
show which variables, processes, and years are affected by a future change. This helps determine which
simulations, figures, tables, or calibrations need to be repeated.

Validation

• Test suite: 8 passed
• The maintained numerical scenario matches the reference exactly.
• Wheel-content verification passed.
• No scientific model routine or equation was changed.

The current reference covers one long glacier-evolution scenario. Additional synthetic edge-case and
calibration tests will be added separately.