fix(safety): verify in-place writes against full content, not a 4-byte prefix

The in-place compression path verifies its readback against a 4-byte magic prefix, while compress_bytes compares full content. Any corruption past the first four bytes passes verification.

safety.rs builds a magic_prefix snapshot and checks that on readback. compress_bytes uses readback_matches against a full pre-apply snapshot. The two paths make different guarantees, and the weaker one is the one that mutates a file in place.

A fork of this engine in abitious used the full-content oracle on both paths. That fork is now deleted in favor of depending on this crate, which is why the difference matters: consumers moving onto the published crate get the weaker check.

Suggested fix: use readback_matches against the full pre-apply snapshot on the in-place path too, or document why a prefix check is sufficient there.

Start in safety.rs and compare the in-place compression readback check with compress_bytes, focusing on magic_prefix and readback_matches. Trace how the full pre-apply snapshot is created and used; the work is done when both paths verify the complete original content rather than only a four-byte prefix.