fix: copy va_list before formatter retries

[sit-d]

Summary

• copy each variadic argument list before the first formatting attempt in errnfo, modsnap, and wchain
• copy the original list again before the dynamic-buffer fallback so truncated output does not reuse an advanced va_list

Verification

• python3 verifier confirmed the touched formatter blocks use va_copy/va_end for the stack and fallback formatting attempts
• git diff --check HEAD~1..HEAD passed

Notes

• Full Windows builds were not run because the Windows SDK/MSVC toolchain is required.