RUSTSEC-2026-0173: proc-macro-error2 is unmaintained #708

As of RUSTSEC-2026-0173, proc-macro-error2 has been declared unmaintained and its repository has been archived.

The RUSTSEC advisory lists manyhow and proc-macro2-diagnostics as alternatives, but both require rewriting how errors are reported in your proc macros. proc-macro-error3 is a maintained fork and drop-in replacement — same API, no code changes beyond the dependency rename, so the switch is minimal and low-risk.

I maintain proc-macro-error3 and use it in my projects, mainly https://github.com/gungraun/gungraun. Would you be open to this switch? I'm happy to submit a PR if that would help.