Sign-in no longer ends with a bare "engine exited unexpectedly" when Windows kills Apple's device-provisioning emulator (issue #8).
That emulator is native code that JIT-compiles into memory. Windows Exploit protection (Control Flow Guard, hardware shadow stacks, dynamic-code blocking), or a security product enforcing the same, can end the process with no traceback. It now runs in its own helper process, so that ends one sign-in attempt instead of the whole engine. The red message gives the helper's exit code, quotes what it last printed, and names the Exploit protection settings to switch off for engine\python\python.exe.
This explains the failure; it does not change what Windows allows. If you see the new message, turn off the listed mitigations for that program and sign in again.
Verified: the crash reproduced locally by starting the helper under Control Flow Guard, hardware shadow stack and dynamic-code blocking, with the engine surviving each and reporting the exit code. 550 pytest + 789 Flutter (6 skip) = 1,339. Installed on Windows 11 as a first install: sign-in reaches Apple and returns its normal answer, the app exits with code 0, and no engine or helper process survives a close or a hard kill of the engine. CI on windows-latest: engine pytest, Flutter analyze + tests, installer build.
Not verified: a full sign-in through 2FA and a sideload on this build.
The installer is unsigned, so SmartScreen will warn — the SHA-256 is in SHA256SUMS.txt.