Support reproducible XZ Linux archives without changing runtime contents

docs/BUILDING.md

@@ -133,6 +133,7 @@ an unsigned headless development candidate:
 
 ```sh
 python3 tools/package_linux.py
+python3 tools/package_linux.py --compression xz
 python3 tools/embeddinggemma2/linux_smoke.py --worker .searchi/runtime/auto/searchi-gemma-worker --models .searchi/models/embeddinggemma2 --output .searchi/packages/linux-worker-smoke.json
 python3 tools/embeddinggemma2/linux_smoke.py --worker .searchi/runtime/auto/searchi-gemma-worker --models .searchi/models/embeddinggemma2 --output .searchi/packages/linux-baseline-smoke.json --baseline
 ```
@@ -148,6 +149,11 @@ They do not exercise GPU providers or certify older physical CPUs, installers
 or desktop behavior. Archive extraction and installed CLI search must also
 be verified before a release gate is closed.
 
+The default archive uses gzip; `--compression xz` produces `.tar.xz` with the
+same normalized TAR members, file checksums and executable modes. XZ uses the
+standard library's preset 6. The extraction smoke accepts either format. A
+smaller download does not change runtime behavior or ABI requirements.
+
 The optional Linux missing-loader check requires explicit root execution and
 util-linux mount namespaces/chroot. It never escalates privileges and verifies
 a private namespace before creating an isolated filesystem: