fix: clear GCC warnings in libc, libkernel, the AGC driver and two tests

[milescoviello]

What

A clean Release build (CMAKE_BUILD_TYPE=Release, BUILD_TESTING=ON) with GCC 13.3 warns in these five places in core/. Each is fixed without changing behaviour, except the null log argument, which was undefined:

• libc/src/General.cpp (chdir): std::filesystem::u8path is deprecated in C++20. The path is now built with the char8_t constructor, as OpenInDirectory_nid_no_patch and the other UTF-8 paths in libc and libkernel already do. Both read the string as UTF-8.
• libkernel/File/src/Open.cpp (sceKernelOpen): the log line formats path with %s before anything rejects a null path. Passing a null pointer to %s is undefined (GCC: "'%s' directive argument is null"). The log now shows (null) for it; the call's result doesn't change.
• libSceAgcDriver/Execution/src/SubmissionCapture.cpp (QueueType): "queue 0x%x is neither the graphics queue nor a compute queue" needs up to 67 bytes, and it was formatted into 64. So for a queue number above 0xfffff, the end of the exception message was cut off. The buffer is now 96 bytes.
• libSceAgcDriver/tests/ConstantPhi.cpp: the result of a [[nodiscard]] IrBuilder::Emit was ignored. It is now static_cast<void>, as the other tests do.
• tests/GuestMemoryModule.cpp: guestMemoryModuleData was both initialized and declared extern ("initialized and declared 'extern'"). The definition is now inside an extern "C" { } block, which gives the same C linkage and external symbol.

Not in this PR:

• DepthSurface.cpp's tile mode format (fix(agc): pass the tile mode of a mismatched depth plane copy as an int #7504) and StagedClearSampledRead.cpp's %u (test(agc): print the staged clear's expected byte count as a size_t #7415) are open as their own PRs.
• The stencil message buffer in State.cpp also warns (-Wformat-truncation). Its values are bytes, so it fits; I left it alone because fix(agc): replace with a stencil operation value the pixel shader exports #6762 changes that buffer.
• sceRtcFormatRFC2822 formats into the guest's 32-byte buffer, which its longest valid string exactly fills, so that warning is a false positive I can't fix by enlarging anything.
• The rest of the warnings are inside stl_algobase.h (GCC 13 -Wstringop-overflow/-Warray-bounds false positives on vector copies) or in glslang.

Tested

Ubuntu 24.04 (Linux 6.8), GCC 13.3, NVIDIA RTX 4070 Ti, driver 595.99.02.

• A clean Release build of main 5bf2e13 with this PR: none of the five warnings above appear.
• ctest -R "^guest_|constant_phi|submission|stencil|graphics|libc": 436/436 pass.
• tools/check_conventions.py --base upstream/main passes. tools/pr_overlap.py lists only conflicting PRs that it could not compare: feat(aot): consolidate native ARM64 compiler, runtime and consumers #5021, test: organize library tests and label CTest suites #6044, Macos m4 #6222 and feat(wasm): run relinked titles in the browser with WebGPU #6879. Of those, test: organize library tests and label CTest suites #6044, Macos m4 #6222 and feat(wasm): run relinked titles in the browser with WebGPU #6879 don't touch these lines; feat(aot): consolidate native ARM64 compiler, runtime and consumers #5021's diff is too large for the API.

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 (NotImplemented_nid_no_patch); silent stubs are listed under Silent stubs in their library's technical debt file
• No notes, investigation .md files or images added to the repository (attach them to this PR)
• New third-party code is a submodule built from source
• Generic behaviour, not specific to one title
• Depends on: none
• Fixes: none
• Unblocks: none
• GPUs tested: NVIDIA GeForce RTX 4070 Ti, driver 595.99.02, Ubuntu 24.04 (Linux 6.8)
• AI-assisted: yes (Claude Code)

🤖 Generated with Claude Code

[MarcZX10]

Reviewed the five warning fixes against their call sites and existing UTF-8 path construction. The null-path log now avoids passing a null pointer to %s; the larger queue message buffer accommodates the full formatted diagnostic; and the ignored nodiscard result and C-linkage definition changes are appropriate. The focused hosted checks and cross-platform relinker/context jobs pass. I did not rerun the full build locally.

[github-actions]

Some comparisons could not be completed because a pull request conflicts with the base branch. Review these shared files manually before concluding that there is no overlap.

Pull request	Shared files not compared
PR 5021	core/libs/prx/libc/src/General.cpp core/libs/prx/libkernel/File/src/Open.cpp
PR 6044	core/libs/tests/GuestMemoryModule.cpp
PR 6226	core/libs/prx/libc/src/General.cpp
PR 6879	core/libs/prx/libc/src/General.cpp
PR 6936	core/libs/prx/libkernel/File/src/Open.cpp
PR 6942	core/libs/prx/libkernel/File/src/Open.cpp
PR 7201	core/libs/prx/libc/src/General.cpp core/libs/prx/libkernel/File/src/Open.cpp
PR 7332	core/libs/prx/libc/src/General.cpp core/libs/prx/libkernel/File/src/Open.cpp