feat(libc): implement the double nextafter export

[Prophet069]

What

Provide the missing double-precision nextafter guest export through std::nextafter. Tests check adjacent values, signed zero, NaNs, infinities, subnormal boundaries, errno and floating-point exceptions. This intentionally does not add nextafterf, which is already proposed in #2934.

Tested

Windows: affected libc built from this branch with a standalone CMake driver using the repository libc target; functional CTest guest_nextafter passed. Linux/macOS and full-project CI not run. This supersedes only the double-precision portion of the closed #1806; no upstream runtime-crash claim is made.

Base: 3cb317fcfd877f93b0b65adbe7b737ba1ba5123f; candidate: f50a480dc22f8334ef4b21f40251d58b86f5a268. python tools/check_conventions.py --base origin/main passed.

Checklist

• Based on current main; no other open PR implements the same functions
• One topic per PR; follow-ups go in a new PR
• No comments in code except technical debt
• Unimplemented paths fail; no silent success stub added
• No investigation files, game data, dumps, logs or images in the branch
• No new third-party dependency
• Generic behaviour, not specific to one title
• Depends on: none
• AI-assisted: yes. Prepared and tested with Codex.

Tests were run on1476f769 and retained across the conflict-free rebase to3cb317fc. That upstream interval does not change the tested libc/decoder/IR sources. No game was started for this submission.

[github-actions]

Thank you for your interest in the project! We are sorry, but we cannot accept more PRs while we work through the existing backlog. This PR has been automatically closed and labeled "intake paused". This is not a judgment of the quality of your changes.

[github-actions]

Progress report (3cb317f → f50a480)

➖ System libraries: 84.48% (+0.0%, +1 implemented functions)

✅ 1 implemented

Library	Name
libc	nextafter_nid_postfix