feat: install addons as Magisk modules when root is active

[Shmayro]

Re-targets the Magisk-module work from #49 onto main. #49 was originally stacked on #48 and got merged into the intermediate refactor/self-contained-installs branch instead of main — so its content never landed. Now that #50 (the refactor) is on main, this is the same commit cherry-picked unchanged on top.

Why

rootAVD only patches the ramdisk. The Magisk app's first-launch "additional setup" prompt is what normally populates /data/adb/magisk/ with the actual binaries (magisk64, magiskinit, magiskboot, magiskpolicy, busybox, stub.apk, …) — without that step, Magisk reports "environment incomplete, abort" on every boot and refuses to mount modules. That means modules dropped under /data/adb/modules/<id>/ are silently ignored.

Without this PR:

• Addon files have to be pushed to /system directly, polluting the qcow2 system overlay.
• Once Magisk is live, /system is wrapped in a read-only magic mount and any subsequent push-based install fails (the INSTALL_FAILED you'd hit if you flip a flag on an already-rooted container).
• User has to open the Magisk app and tap "additional setup" before modules work.

Changes

install_root

• adb root before any /data/adb/* write (the previous omission silently failed every write as the shell user).
• After rootAVD.sh patches the ramdisk, reproduce the Magisk app's FixEnv: extract Magisk.zip's lib/x86_64/ and assets/, rename lib<name>.so → <name> (same naming the Magisk updater-script does), drop app-only files (bootctl, main.jar, module_installer.sh, uninstaller.sh), chmod -R 755. /data/adb/magisk/ is then complete when the patched ramdisk activates on the next QEMU restart — no manual setup prompt.

install_gapps / install_arm_translation

• New _magisk_module siblings to the existing _system paths. They write the payload under /data/adb/modules/<id>/system/ plus a module.prop.
• For ARM specifically: a system.prop with the ABI / native-bridge properties (Magisk's resetprop overrides the ro.* first-write-wins rule) and a post-fs-data.sh that registers binfmt_misc (init.rc files injected via Magisk overlay aren't parsed by init).
• Each branches on a tightened magisk_active() — now checks for the magiskinit binary, not just the directory, so the rootAVD-empty case correctly evaluates to "not yet ready".

Main flow ordering

• install_root runs before install_gapps and install_arm_translation. When ROOT is enabled, the magisk-env is complete by the time the other two run, so they automatically take the module path.

Result

Verified end-to-end on a fresh AVD with ROOT_SETUP=GAPPS_SETUP=ARM_TRANSLATION=1:

• /data/adb/magisk/ populated with magisk64, magiskinit, magiskboot, magiskpolicy, busybox, stub.apk, etc.
• /data/adb/modules/ contains gapps/ and ndk_translation/
• adb shell getprop ro.product.cpu.abilist → x86_64,x86,arm64-v8a,armeabi-v7a,armeabi
• adb shell ls /system/priv-app/ shows Google services (GoogleBackupTransport, GoogleExtServices, …) via Magisk's overlay
• adb install v2rayNG_2.1.7_arm64-v8a.apk succeeds and the app launches cleanly (both com.v2ray.ang and com.v2ray.ang:bg processes alive, no SIGABRT in native_bridge_initialize)
• No "Additional setup required" prompt when opening the Magisk app

Supersedes

• feat: install addons as Magisk modules when root is active #49 (merged into the wrong base, content was never propagated to main)