Fix x86_64 injection: Red Zone adjustment and fallback logic (#91)

- Strictly adhere to the System V AMD64 ABI by skipping the 128-byte "Red Zone" before modifying the stack, see page 23 of https://gitlab.com/x86-psABIs/x86-64-ABI/-/jobs/artifacts/master/raw/x86-64-ABI/abi.pdf?job=build for details.

- Added `inject_via_staging` as a fallback strategy:
  1. Copies the payload to `/data/local/tmp`.
  2. Sets permissions/context (`u:object_r:system_file:s0`).
  3. Loads via standard `dlopen`.
  4. Immediately unlinks the file for stealth.

- Introduced `RegisterRestorer` RAII class to guarantee original registers are restored even if the injection logic returns early due to error.

module/sepolicy.rule

@@ -1,4 +1,5 @@
 allow keystore system_file unix_dgram_socket *
 allow system_file keystore unix_dgram_socket *
 allow keystore system_file file *
+allow keystore shell_data_file dir *
 allow crash_dump keystore process *