Replace DirectSolverGive macros with type-safe functions

[julianlitz]

Summary

This PR replaces the macros with inline functions to improve type safety and code maintainability without sacrificing performance. This is part of the broader effort to eliminate macro usage throughout GMGPolar.

Motivation

As discussed in #149, macros are error-prone and bypass C++'s type system, making bugs harder to catch at compile time. Macros in GMGPolar are currently used for code inlining, but modern C++ compilers handle inline functions just as efficiently in release builds while providing:

• Type safety: Compile-time type checking prevents misuse
• Better debugging: Functions appear in debuggers and stack traces
• Scope safety: No unintended name collisions or side effects

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.12%. Comparing base (eaf5d34) to head (dddd51f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #168      +/-   ##
==========================================
+ Coverage   89.41%   91.12%   +1.71%     
==========================================
  Files          91       91              
  Lines        5241     6254    +1013     
==========================================
+ Hits         4686     5699    +1013     
  Misses        555      555              

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[EmilyBourne]

I wonder if we should make DirBC_Interior_ constexpr in the future. Do you have an opinion?

[julianlitz]

I am not that experienced with constexpr yet, but if you like to make this modification feel free to.

I would guess there is no performance gain due to the branch predictor disregarding the other path.

[EmilyBourne]

I would guess there is no performance gain due to the branch predictor disregarding the other path.

I have never studied the branch predictor in detail but you may well be right, especially after the first iteration.

I am not that experienced with constexpr yet, but if you like to make this modification feel free to.

In any case this would be for a separate PR