#2159 An interrupted engine update leaves BUILD.json.prev and no BUILD.json; UPDATE.bat then skips the engine silently

Descrição

On a Windows/NVIDIA install, UPDATE.bat kept saying `Strata is updated (engine 0.1.39)` although `MIN_ENGINE` is 0.1.43.1. No update was attempted and no reason was given.

**State:** `engine/` had `BUILD.json.prev` (0.1.39) and the 0.1.39 exes, but no `BUILD.json`. The folder was last changed during an earlier update that was interrupted (most likely by a PC crash/reboot).

**Cause:** `get_prebuilt(updating=True)` moves `BUILD.json` to `BUILD.json.prev` before downloading. `update_installed_engine` puts it back only when the update fails inside the process (`new is None`). A crash, reboot, closed window, Ctrl+C or `SystemExit` (only `Exception` is caught) leaves it moved aside. On every later run, `update_installed_engine` hits `if not info.exists() ...: return` and silently skips the engine, so the install never updates again.

**Workaround (confirmed):** with the server stopped, `Move-Item engine\BUILD.json.prev engine\BUILD.json`, then UPDATE.bat. It then updated normally.

Example of failure, and work-around:

<img width="843" height="793" alt="Image" src="https://github.com/user-attachments/assets/80671fed-1f08-445a-9e8a-31d2945f0d6b" />

**Suggested fix:** when `BUILD.json` is missing but `BUILD.json.prev` and the exe exist, restore it (and say so) before checking the version. Optionally put it back in a `finally` around the download as well.