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.

Replaces the obsolete base of closed #2963, which was automatically closed for intake pause. Closed alternatives #4133/#4146 remain unmerged; this draft is for reviewing our existing PR rather than opening another duplicate.

Tested

Windows 10 Pro 22H2 x64 (build 19045), MinGW GCC 15.2.0: built the complete candidate libc.prx from this branch and ran the functional guest_nextafter regression successfully. Tests use synthetic guest-ABI inputs; no game startup, Linux/macOS build or maintainer CI run is claimed.

Based on main dacd78525e537a779682032c0ace7f7af120af05; candidate 036b7720c33b9e05f8984c10614a7293dc4d08e2. No dumps, runtime packages, investigation reports or images are included. No speed improvement is claimed.

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 throw; no silent success stub added
• No notes, investigation .md files or images added to the repository
• New third-party code: none
• Generic behaviour, not specific to one title
• Depends on: none
• Unblocks: none (no clean-main Game bring-up claim)
• GPUs tested: n/a (libc change)
• AI-assisted: yes; prepared with Codex; the submitting author is responsible for every line

[github-actions]

Progress report (dacd785 → 036b772)

➖ System libraries: 88.15% (+0.0%, +1 implemented functions)

✅ 1 implemented

Library	Name
libc	nextafter_nid_postfix

[johnohhh1]

Independent Linux validation of 036b7720c33b9e05f8984c10614a7293dc4d08e2, cleanly cherry-picked onto main a0f5916a293067ca840e835e58a8a55a0b764b4b, retaining the original author and stable patch ID.

Ubuntu 26.04.1 x86-64, GCC 15.2.0, Release, bundled dependencies, BUILD_TESTING=ON.

• Before the patch, your GuestNextafter.cpp regression fails to link against baseline libc: undefined nextafter_nid_postfix.
• With the patch, the full Linux build and six selected suites pass without skips: guest_nextafter, guest_math, guest_wide_conversion, guest_wide_strings, guest_strings, patched_libraries. Serial execution, 30-second per-test limit, 0.06 seconds total. Conventions and whitespace checks pass.
• A separate guest-ABI probe passes 19,488 functional cases across all four rounding modes and three initial errno values. Non-NaN expected values are computed by an independent binary64 bit-stepping oracle, without calling host nextafter for the expected result. Cases include both signed zeros, adjacent values, normal/subnormal boundaries, finite extremes, infinities, deterministic randomized inputs, and quiet/signaling NaNs. It checks preserved errno for normal results, preservation of pre-existing exception flags, and signaling-NaN invalid exceptions. No failures in 74,380 assertions; approximately 0.008 seconds. The PR's own regression also passes its overflow/underflow errno and exception checks.

The value oracle follows C11 draft N1570, sections 7.12.11.3 and F.10.8.3. These are synthetic Linux checks, not console measurements or a title run. No additional Windows/macOS testing or full CTest-suite claim.

AI-assisted validation/report: Codex. No changes were made to your implementation.

[CnRJay]

The implementation itself looks fine. The description doesn't name a title that imports nextafter, though. Since #1205/#1208/#950, reviewers ask new-export PRs for the title ID and the log line where that title stops at the export on main (FAIL: unresolved ELF import ... from libc.prx), and export-only PRs without one have been closed. If you know of a title that reaches it (tools/import_audit.py over a title's imports will show it), adding that to the description should be all that's missing.

[oneandonlydean]

Closing: no title is known to reach this. With the queue at 700+ open PRs, please hold off on new PRs that don't name a title needing the change until it's down. If a title stops on this on current main, reopen with the title ID and the log line.

[Prophet069]

PPSA02015 reaches it