Two stages tested separately, because a global failure says nothing about where: PBKDF2 against the known derived key - which also runs on Win64, so that stage validates in seconds without deploying to a phone - then AES-GCM against the plaintext hash, Android only.
The vector is embedded as a unit rather than read from a file: no network, no deployment, so a first-run failure can only come from the decryption itself. Work runs on a thread; 600k HMAC rounds on the UI thread would have Android kill the app for not responding.
PBKDF2 is implemented in Pascal rather than through javax.crypto SecretKeyFactory: PBEKeySpec takes a char[] and the provider decides how those chars become bytes, so a non-ASCII sync password would derive a different key than the browser and fail as 'wrong password'. AES-GCM does go through JNI - one call, hardware-backed, and WebCrypto's tag-appended layout is exactly what Cipher expects.
The vector is generated with WebCrypto itself and uses a non-ASCII password on purpose; an ASCII-only vector would have hidden that bug. It records the derived key separately from the plaintext so a failure points at one stage instead of the whole chain.
r-zakarya