fix(ci): clone submodule content in rattler-build package

rattler-build's source.path copy does not include submodule contents —
directories like mods/silo/, mods/solver/, mods/gears/, and mods/datums/
end up empty in the work directory, causing cmake --install to fail
when it tries to install files from those paths.

Add a clone_if_empty helper to build.sh that detects empty submodule
directories and shallow-clones them from their remotes before CMake
runs. Add git to build requirements in recipe.yaml.

This fixes the release build (AppImage) failure where cmake --install
failed at the silo addon install step after successfully installing
the silo tree-node icons from src/Mod/Create/resources/.

package/rattler-build/build.sh

@@ -11,6 +11,24 @@ mkdir -p "$CCACHE_DIR"
 echo "ccache config: CCACHE_DIR=$CCACHE_DIR CCACHE_BASEDIR=$CCACHE_BASEDIR"
 ccache -z || true
 
+# rattler-build's source.path does not copy submodule contents — clone them
+# if the directories are missing or empty.
+clone_if_empty() {
+    local dir="$1" url="$2" ref="$3"
+    local count
+    count=$(find "${dir}" -mindepth 1 -not -name '.git' 2>/dev/null | head -1 | wc -l)
+    if [ "$count" -eq 0 ]; then
+        echo "Submodule ${dir} missing content — cloning from ${url} (${ref})"
+        rm -rf "${dir}"
+        git clone --depth 1 --branch "${ref}" --recurse-submodules "${url}" "${dir}"
+    fi
+}
+
+clone_if_empty mods/silo   https://git.kindred-systems.com/kindred/silo-mod.git main
+clone_if_empty mods/solver  https://git.kindred-systems.com/kindred/solver.git   main
+clone_if_empty mods/gears   https://git.kindred-systems.com/kindred/gears.git    main
+clone_if_empty mods/datums  https://git.kindred-systems.com/kindred/datums.git   main
+
 if [[ ${HOST} =~ .*linux.*  ]]; then
     CMAKE_PRESET=conda-linux-release
 fi

package/rattler-build/recipe.yaml

@@ -16,6 +16,7 @@ requirements:
   build:
     - ccache
     - cmake
+    - git
     - compilers>=1.10,<1.11
     - doxygen
     - icu>=75,<76