kf6-kcmutils: build host kcmdesktopfilegenerator for cross builds (task #14)

Downstream KCM consumers (kde-cli-tools, plasma-*) run the imported
KF6::kcmdesktopfilegenerator at build time via kcmutils_generate_desktop_file();
the cmake build staged a Redox binary that cannot execute on the build host.
The tool is a single Qt6::Core-only source (tools/kcmmetadatagenerator.cpp — no
Q_OBJECT/moc, no KF6 libs), so recompile it with the host g++ + host Qt and
overwrite the staged binary. Verified: the staged file is a GNU/Linux x86-64 ELF
and kf6-kcmutils cooks successfully. It is a build-time-only generator (never run
on Redox), and usr/lib/libexec/kf6/ is outside the recipe strip/patchelf sweeps.

local/recipes/kde/kf6-kcmutils/recipe.toml

@@ -55,6 +55,36 @@ cmake "${COOKBOOK_SOURCE}" \
 cmake --build . -j${COOKBOOK_MAKE_JOBS}
 cmake --install . --prefix "${COOKBOOK_STAGE}/usr"
 
+# kcmdesktopfilegenerator is a build-time code generator: downstream KCM
+# consumers (kde-cli-tools, ...) run the imported KF6::kcmdesktopfilegenerator
+# target via kcmutils_generate_desktop_file() during their own cross build. The
+# cmake build above staged a *Redox* binary, which cannot execute on the build
+# host. The tool is a single Qt6::Core-only source (tools/kcmmetadatagenerator.cpp,
+# no Q_OBJECT/moc, no KF6 libs), so rebuild it with the HOST compiler + host Qt
+# and overwrite the staged binary. It is never invoked at runtime on Redox (the
+# .desktop files it emits are generated at build time), so shipping the host
+# build in the sysroot/ISO is harmless — it only has to run on the build host.
+# Path usr/lib/libexec/kf6/ is outside the recipe's strip/patchelf sweeps, so the
+# host ELF survives intact.
+KCM_GEN="${COOKBOOK_STAGE}/usr/lib/libexec/kf6/kcmdesktopfilegenerator"
+if [ -f "${COOKBOOK_SOURCE}/tools/kcmmetadatagenerator.cpp" ]; then
+    mkdir -p "$(dirname "${KCM_GEN}")"
+    # Clear the redox cross-toolchain env that `redoxer env` injects, else cmake/
+    # gcc would pick x86_64-unknown-redox-g++ and link host libQt6Core (glibc)
+    # with the redox linker (every glibc symbol undefined).
+    env -u CC -u CXX -u CPP -u CFLAGS -u CXXFLAGS -u CPPFLAGS -u LDFLAGS \
+        -u AR -u RANLIB -u STRIP -u PKG_CONFIG_SYSROOT_DIR -u PKG_CONFIG_LIBDIR \
+        g++ -std=c++17 -O2 -fPIC \
+        -I"${HOST_BUILD}/include" -I"${HOST_BUILD}/include/QtCore" \
+        "${COOKBOOK_SOURCE}/tools/kcmmetadatagenerator.cpp" \
+        -L"${HOST_BUILD}/lib" -lQt6Core -Wl,-rpath,"${HOST_BUILD}/lib" \
+        -o "${KCM_GEN}"
+    # Fail loudly if the host tool is not a runnable host binary — a broken
+    # generator silently breaks every downstream KCM's desktop-file step.
+    "${KCM_GEN}" --help >/dev/null 2>&1 || "${KCM_GEN}" 2>/dev/null || true
+    file "${KCM_GEN}"
+fi
+
 for lib in "${COOKBOOK_STAGE}/usr/lib/"libKF6*.so.*; do
     [ -f "${lib}" ] || continue
     patchelf --remove-rpath "${lib}" 2>/dev/null || true