Replace exported settings globals with typed accessors

[paulromano]

Description

Continuing toward Windows support — this PR is aimed at avoiding the need for Windows-specific exports (similar to #4065). In this case, the Python bindings currently read and write mutable OpenMC settings directly with ctypes.in_dll, which requires implementation details to be exported from the shared library. This change replaces those data accesses with name-based C API getter and setter pairs for boolean, 32-bit integer, 64-bit integer, and double settings, along with a read-only string getter. An alternative would be to have a get/set pair for each setting specifically, but that has the downside of requiring more functions in the long term. This design allows us to handle future settings without introducing new C API functions. The existing openmc.lib.settings properties remain unchanged for users while their ctypes implementation is reduced to one accessor pair per datatype.

One other change I made here is to move the body of int main() into a dedicated openmc_main entry point within libopenmc, and the installed executable simply delegates to it. This again helps us avoid exporting global symbols just for Windows.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)