fix(libkernel): return guest chmod errors and implement sceKernelFutimes

[tototomate123]

What

Split from #1616, following its review. Return guest errors from kernel chmod and POSIX -1/guest errno from POSIX chmod. Implement sceKernelFutimes while retaining POSIX futimes behavior and timestamp validation. Two commits.

Tested

Linux x86-64, GCC 16.2.1, Release. guest_filesystem passes, including chmod failures, descriptor timestamps and invalid microseconds. Conventions and whitespace checks pass on current main (5edae353).

Depends on

None for build. HFW integration also needs #2785 (relinker), #2786 (loader), #2788 (http), #2789 (apr), #2790 (shader). #1269 (media), #1339 (VRR), #1586 (unwind) and #1436 (imports) are now on main. Local image-table experiments are excluded; open #757 → #758 overlaps them, and #920 is closed without merging. Earlier combined integration reached menu/loading UI; these isolated branches have not been game-tested and gameplay is unverified.

Checklist

• Based on current main; no duplicate implementation found beyond superseded fix(linux): resolve loader and APR failures exposed by HFW #1616
• One topic per PR
• No code comments except technical debt
• Unimplemented paths throw; offline HTTP limitations documented
• No investigation notes, logs or images committed
• No new third-party dependencies
• Generic behavior; no title-specific dispatch
• Depends on: none for build; HFW integration requirements listed above
• AI-assisted: yes, Codex

[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 (5edae35 → 9107be4)

➖ System libraries: 84.52% (+0.0%, +1 implemented functions)

✅ 1 implemented

Library	Name
libkernel	sceKernelFutimes

[adriadam10]

Checked the chmod/futimes error paths against the guest test (guest_filesystem, incl. null path, missing path, EBADF, invalid usec) and the partial build + ctest (94 tests) on Linux; the new test does not link without the change. Merging once CI is green.