chore: ignore Arch packaging build artifacts

A local `scripts/build-arch.sh` run leaves a vendored cargo cache
(`.cargo-arch/`), a makepkg workdir (`packaging/arch/pkg/`, `src/`) and the
built package in the tree — tens of thousands of untracked files that bury real
changes in `git status`.

.gitignore

@@ -64,3 +64,9 @@ src-tauri/.cargo/config.toml
 /docs/README.md
 /docs/api-redirect.md
 /docs-site/book/
+
+# Arch packaging build artifacts (vendored cargo cache, makepkg workdir, output package)
+/.cargo-arch/
+/packaging/arch/pkg/
+/packaging/arch/src/
+/packaging/arch/*.pkg.tar.zst