>... have the OS generate a hash each time the file changes...

I'm not sure I would want this either tbh. If I have a 10GB file on my filesystem, and I want to fseek to a specific position in the file and just change a single byte, I would probably not want it to re-hash the entire file, which will probably take a minute longer compared to not hashing the file. (Or maybe it's fine and it's fast enough on modern systems to do this every time a file is modified by any program running, I don't know how much this would impact the performance.).

Perhaps a higher resolution timestamp by the OS might help though, for decreasing the chance of a file having the exact same timestamp (unless it was specifically crafted to have been so).