feat: add configurable gradient threshold to ABF and ABFPlusPlus (A1)

[csparker247]

Summary

• Adds setGradientThreshold(T) method to both ABF and ABFPlusPlus alongside the existing setMaxIterations()
• Adds gradThreshold parameter (defaulting to T(0.001)) to the full static Compute() overload on both classes
• Replaces the hardcoded 0.001 literal in the solver while loop with the configurable parameter
• Fully backward-compatible: default value preserves existing behavior

Test plan

• All existing tests pass unchanged (10/10 parameterization tests pass)
• ABFPlusPlus_TightThreshold: tightening threshold to 1e-6 produces a lower final gradient than the default
• ABFPlusPlus_LooseThreshold: threshold of 1e6 (above initial gradient) causes zero solver iterations

Closes #15

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