maint(mac): support for xcode 26.4+ (clang 21.0)

Currently, Keyman Core fails to build with clang 21.0, so we are restricted to xcode 26.3.

When we make the update presumably in 20.0 alpha, we'll need to back-port to current stable so we can continue to make stable builds on the build agents with newer versions of xcode.

No files or tests are named. Start by reproducing the Keyman Core build with Xcode 26.4 or clang 21.0 and identify the failing component; done means the build works with the newer toolchain and the fix is back-ported to the current stable branch.

Written by the indexing model from the issue text.