feat: C++20 modules support

[wdconinc]

This PR adds C++20 modules support (see also AIDASoft/podio#907). This mainly involved two things:

• TU-local entities should be removed from headers (since they have internal linkage and we want external linkage), which in this case means moving from static constexpr to inline constexpr,
• CMake needs support to install the right files, when SIO_ENABLE_CXX_MODULES=ON (and CMAKE_CXX_STANDARD=20 or newer).

BEGINRELEASENOTES

• feat: add C++20 modules support

ENDRELEASENOTES

[Copilot]

Pull request overview

This PR adds C++20 modules support to the SIO library, enabling modern C++ module-based compilation when SIO_ENABLE_CXX_MODULES=ON and using C++20 or newer. The implementation focuses on two main areas: converting translation-unit-local entities to have external linkage by changing from static constexpr to inline constexpr, and adding CMake infrastructure to build and install module interface files.

• Changed constant definitions from static constexpr to inline constexpr for C++20 module compatibility
• Added CMake module support with compiler/version/generator checks
• Created module interface file (sio_module.ixx) to export SIO types and APIs

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
source/sio_module.ixx	New module interface file that declares and exports SIO types, constants, and classes for C++20 module consumers
source/include/sio/definitions.h	Updated constant declarations from static constexpr to inline constexpr to ensure external linkage in modules
source/include/sio/memcpy.h	Updated sizeof_helper template members from static constexpr to inline static constexpr for module compatibility
source/CMakeLists.txt	Added conditional installation logic to install module files when SIO_ENABLE_CXX_MODULES is enabled
cmake/SIOModules.cmake	New CMake module providing functions and validation for C++20 module support (CMake 3.29+, GCC 14+, Clang 18+, Ninja generator)
CMakeLists.txt	Includes SIOModules.cmake and conditionally installs builtin ZLIB when modules are enabled

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[tmadlener]

Looks good to me, but I have no easy way to test this in depth at the moment. Similar to the one in podio, I would make a new tag first and then merge this afterwards.