Update memory file modification times after writes

[x0Lazarus]

Appending to or overwriting an existing memory file changes its contents, but fs.modified(path) still returns the time from its last commit. Truncating the file has the same problem. Code checking modification times can therefore miss a change.

Update the timestamp after successful writes and truncation, while preserving the creation time and the existing return values. writelines uses the same write path, so the timestamp also advances when some lines are written before a later line raises an error. Reads, empty writes, and rejected writes leave it unchanged.

Five regression cases fail before the fix. The memory and mapping tests pass on Windows with Python 3.12: 95 passed and one skipped. Ruff lint and formatting checks and codespell pass for the changed files. The broader filesystem specification tests could not collect in this environment because NumPy is missing; Docker, cloud backends, and downstream projects were not tested.

[martindurant]

I believe this successfully copies posix behaviour