Fix GH-473: Smooth transition for OHM coefficient selection

[sunt05]

Summary

Replaces step-function coefficient selection in OHM with smooth linear blending to eliminate numerical divergence at threshold boundaries.

• Adds temperature transition zone (±2°C around threshold)
• Adds moisture transition zones for surface wetness and soil moisture
• Blends coefficients from all four conditions (summer/winter × wet/dry)
• Adds division-by-zero guard for SoilStoreCap

Context

This addresses the root cause of #473 by eliminating the step function entirely. When there's no discontinuity, floating-point precision differences cannot cause different coefficient regimes to be selected.

Before (step function)

T >= 10°C → summer coefficients
T < 10°C  → winter coefficients

After (smooth blend)

T >= 12°C     → 100% summer coefficients
8°C < T < 12°C → linear blend
T <= 8°C      → 100% winter coefficients

Breaking Change

⚠️ This is a physics change that produces intentionally different results in transition zones (~8-12°C for temperature). This is expected and arguably more physically realistic.

TODO

• Regenerate reference data (sample_output.csv.gz)
• Full test suite validation
• Documentation updates (CHANGELOG, user docs)

Test plan

• Smoke tests pass after reference data regeneration
• Full test suite passes
• Verify cross-platform consistency (macOS/Linux/Windows)

Resolves #473

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