Fix catastrophic cancellation in VanGenuchten near theta_range[0]

[gerlero]

See gerlero/fronts#152

[Copilot]

🟢 Approval recommended

The functional change is targeted and the added tests cover the intended numerical-stability regression.

Pull request overview

This PR improves numerical stability of the Van Genuchten–Mualem relative conductivity computation near residual saturation to avoid catastrophic cancellation, and adds regression tests that validate values and derivatives in this regime.

Changes:

• Rewrites VanGenuchten._kr using log1p/expm1 to improve stability for small effective saturations.
• Adds tests covering Van Genuchten diffusivity and derivatives near residual saturation, plus a float32 _kr regression check.

File summaries

File	Description
src/frontx/models.py	Replaces the cancellation-prone 1 - (1 - x)**m form in _kr with a stable expm1(log1p()) formulation.
tests/test_models.py	Adds regression tests for near-residual behavior and a float32 stability check for _kr.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.25%. Comparing base (fd68425) to head (26c1e70).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #98      +/-   ##
==========================================
+ Coverage   86.05%   86.25%   +0.20%     
==========================================
  Files          13       13              
  Lines         552      553       +1     
==========================================
+ Hits          475      477       +2     
+ Misses         77       76       -1     

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