fix: arch linux uses rustup for wasm target support

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

warzone/scripts/build-linux.sh

@@ -410,7 +410,11 @@ do_local_deps() {
       sudo dnf install -y gcc gcc-c++ make pkg-config openssl-devel curl >/dev/null 2>&1
       ;;
     pacman)
-      sudo pacman -Sy --noconfirm base-devel pkg-config openssl curl >/dev/null 2>&1
+      sudo pacman -Sy --noconfirm base-devel pkg-config openssl curl rustup >/dev/null 2>&1
+      # Arch: ensure rustup manages the toolchain (pacman rust conflicts with rustup)
+      if ! rustup show active-toolchain &>/dev/null; then
+        rustup default stable 2>/dev/null || true
+      fi
       ;;
     brew)
       brew install openssl pkg-config 2>/dev/null || true