fix(autoconfig): replace existing preset contents

[Guflly]

Existing ext4 files were opened with w+, but the vendored filesystem ignored O_TRUNC. Shorter presets kept the previous tail and failed validation.

This honors truncation on open and adds a regression test for replacing a 420-byte file with 382 bytes.

Closes #178

[henla464]

Thank you, super quick solution :)

[SuperKali]

Thanks @Guflly i'll check asap.

[SuperKali]

Tested on a real mke2fs image with /root/.not_logged_in_yet already present: main reproduces #178, this branch writes 382 bytes and validates, partitioned path included. One for later: the freed block never gets cleared in the bitmap, because ext4_bmap_bits_free is called with a count where it expects an end index, so every rewrite leaks one block. resize2fs handles the result fine, we will fix that one on our side.