tar: PAX header can be misapplied to non-file entries

### Summary

When a tar stream contains multiple "header" entries prior to a file entry, tar-rs applies the PAX header (`x`) to the _next_ entry in the stream, regardless of type. For example, a stream of `x -> L -> file` (PAX, GNU longname, file) would result in `x`'s extensions being applied to `L` rather than to `file`.

[Per POSIX pax](https://pubs.opengroup.org/onlinepubs/9799919799/utilities/pax.html), this is incorrect: a PAX header always applies to a file entry, not any intermediary entries. See the "pax Header Block" section for the specific prescription there.

As a result of this, an attacker can contrive a tar containing a sequence of tar headers such that tar-rs applies the PAX header's `size` extension to the next header in sequence, effectively desynchronizing the stream and enabling tar-rs specific skippage/extraction of members. In other words, a file can be contrived to extract differently on tar-rs than on other tar parsers.

### PoC

[This tar](https://github.com/user-attachments/files/27141941/pax-overrides-extension-header.tar.zip) (zipped for size) demonstrates the desynchronization: with `tar tvf`:

```
% tar tvf tests/archives/pax-overrides-extension-header.tar
---------- 0 0 0 2048 Dec 31 1969 longname.txt
---------- 0 0 0 0 Dec 31 1969 file_b
```

with `tar-rs`:

```
---- pax_size_does_not_apply_to_extension_headers stdout ----

thread 'pax_size_does_not_apply_to_extension_headers' (250476889) panicked at tests/all.rs:2121:27:
called `Result::unwrap()` on an `Err` value: Custom { kind: Other, error: "numeric field was not a number: AAAAAAAA when getting cksum for AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA" }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

In the above case, the PoC is not weaponized, so it jumps into the middle of an entry and subsequently fails the checksum test rather than silently continuing with attacker-controlled archive state.

### Impact

This is very similar to GHSA-j5gw-2vrg-8fgx and GHSA-fp55-jw48-c537 in impact -- an attacker can use this to extract (or not extract) files from a tar stream depending on the tar parser used, which in turn can be used to obscure the presence of malicious files.