Affected: npm: tar

## Summary

`node-tar` strips trailing `NUL` bytes from long-name (`L`) and long-linkpath (`K`) GNU extended headers but does **not** apply the same sanitization to equivalent fields delivered via PAX (`x` typeflag) extended headers. A PAX record of the form `path=visible.txt\x00hidden.txt` is parsed verbatim into `entry.path` and flows into `fs.lstat()` / `fs.open()`, which Node.js core rejects with `ERR_INVALID_ARG_VALUE`. The throw originates inside an `FSReqCallback` async chain that is **not** wrapped by the consumer's `await/try-catch` around `tar.x()` — it surfaces as `uncaughtException` and terminates the process.

This is a remote denial-of-service primitive against any process that extracts attacker-supplied tarballs through `tar.x` / `tar.extract` / `tar.t` / `tar.Parser`, even when the consumer follows the documented `try/catch` error-handling pattern.

A secondary parser-differential (CWE-436) exists because `tar(1)`, `bsdtar`, and Python `tarfile` truncate the path at the first `NUL` (yielding `visible.txt`) while node-tar retains the full string. A validator that pre-scans a tarball with one tool and extracts with the other is bypassed.

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## Root cause

### Vulnerable sink — `src/pax.ts:157-183`

PAX KV records flow through `parseKVLine`. The value half (`v`) is assigned directly to the result object with no sanitization for embedded NUL bytes:

```ts // src/pax.ts:157 const parseKVLine = (set: Record<string, unknown>, line: string) => { const n = parseI…