Switch carb absorption model to Parabolic

[Rdeboer180]

Summary

• Change default carbohydrate absorption model from PiecewiseLinear to Parabolic (Scheiner GI curve)
• This provides a smoother, more natural glucose spike curve that better matches physiological carbohydrate absorption

Details

Updated CarbStore.swift in LoopKit submodule to use ParabolicAbsorption() instead of PiecewiseLinearAbsorption().

The parabolic model creates:

• A gentler initial rise (vs. aggressive two-phase spike)
• More natural peak curve
• Smoother decline after peak

Testing

• Review glucose prediction outputs with real patient data
• Compare spike profiles before/after for accuracy
• Validate against continuous glucose monitor readings

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