Bump GSL to v4.2.2 to fix MSVC C4875 on VS2026 toolset

arcana.cpp pins GSL v4.0.0, whose GSL_SUPPRESS macro expands to a bare
identifier [[gsl::suppress(x)]] on MSVC. The VS2026 runner image
(_MSC_VER >= 1950) emits C4875 for that form (non-string-literal argument
to [[gsl::suppress]] is deprecated), and our /WX builds promote it to an
error, breaking every Windows/UWP target.

GSL v4.2.2 switches to the string-literal form for _MSC_VER >= 1950, which
the new toolset accepts. Override the transitive pin here (FetchContent is
first-declaration-wins) so the fixed release is used.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Branimir Karadžić

CMakeLists.txt

@@ -32,6 +32,14 @@ FetchContent_Declare(CMakeExtensions
 FetchContent_Declare(googletest
     URL "https://github.com/google/googletest/archive/refs/tags/v1.17.0.tar.gz"
     EXCLUDE_FROM_ALL)
+# Override the GSL version pinned transitively by arcana.cpp (v4.0.0). Declared
+# here (FetchContent is first-declaration-wins) so the newer release is used.
+# v4.2.2's GSL_SUPPRESS emits a string-literal [[gsl::suppress("...")]] on MSVC
+# toolsets >= 19.50 (VS2026), avoiding C4875 which broke the /WX Windows builds.
+FetchContent_Declare(GSL
+    GIT_REPOSITORY https://github.com/microsoft/GSL.git
+    GIT_TAG v4.2.2
+    EXCLUDE_FROM_ALL)
 FetchContent_Declare(ios-cmake
     GIT_REPOSITORY https://github.com/leetal/ios-cmake.git
     GIT_TAG 4.4.1