perf(android): move state persistence off the UI thread (AN-1)

switchVersion/markSuccess/setUuid/setLocalHashInfo only read/modify
SharedPreferences via a synchronous commit(); they were dispatched to the
UI thread purely to serialize them. markSuccess runs on every cold start,
so its blocking disk write on the main thread caused jank/ANR on low-end
devices.

Add StateSerialRunner (a dedicated single-thread executor) that preserves
the same serialization + commit() persistence guarantees while keeping the
disk I/O off the UI thread. reloadUpdate/restartApp remain on UiThreadRunner
since reload is a genuine UI operation.

Also adds CODE_AUDIT.md documenting the full P0-P2 audit and remediation.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: sunnylqm