chore: bake rootAVD and use tar.gz storage for addon payloads (#52)

Drop the runtime `git clone https://gitlab.com/newbit/rootAVD.git` (the
last remaining external download for the rooting flow). The rootAVD
repo (rootAVD.sh + bundled Magisk.zip) is now downloaded as a tarball
at image build time. While at it, switch the existing ndk_translation
bake to the same shape so both addons store their upstream tar.gz
verbatim at /opt/<name>.tar.gz instead of pre-extracted trees.

install_root, install_arm_translation, and install_gapps all extract
their payload into /tmp/<name>/ at the top of the install, work from
there, and clean up at the end — same pattern across the three.

Why:
- No runtime gitlab.com dependency for rooting (previously 84.7 MB git
  clone with full history, now a ~12 MB tar.gz baked at build time).
- ~26 MB smaller image disk footprint after pull (the upstream tar.gz
  files are still LZMA/gzip-compressed in /opt; we only pay
  decompression on install).
- Lazy work: an addon that isn't enabled never gets extracted.
- Symmetric layout — install_root, install_arm_translation, and
  install_gapps all stage under /tmp/<name>/ now.

Verified end-to-end on a fresh AVD with ROOT_SETUP=GAPPS_SETUP=
ARM_TRANSLATION=1: all four markers land, /data/adb/modules/ contains
gapps and ndk_translation, ABI list advertises arm64-v8a, Google
services visible at /system/priv-app/ via Magisk overlay, and an
arm64-v8a-only APK installs cleanly.