build: define gitsha for swift package builds

[webern]

Human Summary

Basically the CMake stuff doesn't run if you consume the library through the Swift package. So this was the agent solution for that: a file that CMake doesn't glob but that Swift does include, which writes the sha as unknown. This is not ideal. I want to find a better versioning mechanism but everything is hard without a package manager.

Summary

mx::core::gitSha is declared in Version.h and defined by GitSha.cpp, which cmake/GitSha.cmake
renders from Version.cpp.in into the build tree at build time. The Swift package has no code
generation step and simply globs src/, so the Mx product compiles the declaration with no
definition anywhere.

Nothing catches this today. swift build --product Mx only archives a static library, and an
undefined symbol in an archive is not diagnosed until something links an executable against it.
It surfaced when komp switched to consuming mx through the Swift package instead of a prebuilt
xcframework:

Undefined symbols for architecture x86_64:
  "mx::core::gitSha", referenced from:
      mx::core::mxSoftwareAttribution() in Mx.o

This adds GitShaDefault.cpp defining gitSha as "unknown", which is the value Version.h already
documents for a build made outside a git checkout.

CMake builds are unaffected. The mx_core source list is explicit and no glob covers
mx/core/*.cpp, so the generated definition stays the only one there.

Worth considering as a follow-up: the swift CI job could link a small executable rather than only
archiving, so this class of defect cannot go latent again. Left out of this PR because it is a
choice about the workflow rather than part of the fix.

Testing

• CMake api target builds with the new file present, never compiles it, and reports no duplicate symbol
• ar t libmx_core.a lists GitSha.cpp.o and no GitShaDefault.o, so CMake still uses the generated definition
• A consumer program linked against the Swift package objects builds and runs
• Removing only GitShaDefault.cpp.o from that same link fails with undefined mx::core::gitSha, confirming this file is what resolves it
• komp builds and runs on macOS and the iPad simulator against the Swift package

References

• Related: Create an Self-Contained Example that relies on the installation of Mx #78. A consumer built during CI is the kind of check that would have caught this; the Swift package has the same blind spot the issue describes for the CMake install path.