fix(libc): create noreplace streams exclusively - #5949

What

C++ _Fiopen previously implemented noreplace by probing the existing file for reading before opening it for writing. A Windows file shared for writing but not reading makes that probe fail, so the later open can truncate the file. The separate probe also lets concurrent creators both succeed.

Keep Noreplace in the mode key for writing modes and select the existing exclusive fopen modes for its six valid combinations. Read-only modes continue to ignore Noreplace, as the existing guest_fiopen test requires. The combinations follow Microsoft STL's _Fiopen table; fopen already uses CREATE_NEW on Windows and exclusive host opening elsewhere. No separate existence probe remains.

Tested

Original validation at PR head d902afe2d on main e34c47921: Windows 11 Pro 10.0.26300, WinLibs GCC 15.2.0, Release. The write-shared truncation regression failed on main and passed with the fix. The test covered all six exclusive mode combinations, ate, EEXIST for an existing file, and twelve two-creator races with exactly one winner. The full default Windows build and four focused file tests passed.

Follow-up at head 3f95553e8, merged with main 5d9dcc42d:

• Linux (VirtualBox VM): Ubuntu 24.04.5 LTS, kernel 7.0.0-31-generic, GCC 13.3, Release, 8 vCPU / 8 GiB. Full BUILD_TESTING=ON build passed on the fixed source. ctest --test-dir build -R '^(guest_fiopen|file_streams|file_position|guest_posix_filesystem|guest_filesystem)$' --output-on-failure: 4/4 passed; guest_filesystem is not registered on Linux. Before the implementation fix, file_streams passed and guest_fiopen failed with the new assertion. Linux graphics/Vulkan tests: NOT RUN — intentionally excluded from VirtualBox VM validation. Native/bare-metal Linux and native Linux GPU were not tested.
• Windows (native host): Windows 11 Pro 10.0.26300, AMD Ryzen 9 7900X, 31.7 GiB RAM, NVIDIA RTX 4070 Ti SUPER, WinLibs GCC 15.2.0, Release. ctest --test-dir build -R '^(guest_fiopen|file_streams|file_position|guest_filesystem|guest_posix_filesystem)$' --output-on-failure: 5/5 passed. The unmodified full build stops in a current-main AGC test because MinGW rejects alignas(65536) (object-file maximum 8192). A temporary, uncommitted adjustment to that unrelated test allowed the remaining targets to build; it was removed before this commit. Full CTest with that adjustment: 1283 passed, 3 failed, 14 skipped. agc_driver_multisample_fill_host_only passed on serial retry; guest_poll and guest_kqueue repeated their aborts. Their cause remains unverified.
• Conventions and whitespace checks passed. No title or homebrew run. No performance claim.

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 stubs added
• No notes, investigation files or images added to the repository
• New third-party code is a submodule built from source; none added
• Generic behaviour, not specific to one title
• Depends on: none
• Unblocks: none
• GPUs tested: n/a
• AI-assisted: yes, Codex

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