[slopgobblercobbler]

Problem

In Builder::append_pax_extensions, keys containing = or empty keys are accepted without error. When the archive is parsed back, the reader splits on the first = (pax.rs:109), resulting in split corruption where the key is truncated at the first = and the remainder becomes part of the value. For example, ("a=b", b"c") reads back as key: "a", value: "b=c", and ("=", b"") reads back as key: "", value: "=".

According to the POSIX PAX specification (IEEE Std 1003.1), extended header keywords must not contain = and cannot be empty.

Solution

In Builder::append_pax_extensions (src/pax.rs), validate each key before formatting:

• Reject empty keys
• Reject keys containing =
• Reject keys containing newlines or NUL bytes

Added unit test pax_invalid_keys_rejected in tests/all.rs covering these failure paths and ensuring valid keys continue to serialize cleanly.

Fixes #486