internal linkage for constexpr variables for module use

[stephenberry]

Added inline to namespace-scope constexpr variables (giving them external linkage), and removed static from function templates.

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constexpr functions are implicitly inline, but constexpr variables are not — they're implicitly const, and const at namespace scope has internal linkage in C++.

So:

// Internal linkage (each TU gets its own copy)
constexpr int x = 42;

// External linkage (one definition shared across TUs)
inline constexpr int x = 42;

This distinction doesn't matter in traditional header-based code — each TU gets its own identical copy and the linker is fine with it. But with C++ modules, GCC 15 enforces [basic.link]/[module.reach]: a module interface unit must not expose entities with internal linkage (TU-local entities) in any reachable declaration. Since templates and inline functions in the module interface reference these constants, the internal-linkage variables "leak" through, and GCC rightfully rejects it.