fix(libkernel): read and write guest sockets

[SleglJan]

What

read, write, _read and _write on a guest socket descriptor (0x10000000 and up) went to the host as if the number were a host descriptor, so sceKernelRead and sceKernelWrite threw (sceKernelWrite: write failed, fd=…, errno=9) on the first byte a title moved through a socket that way. On the console both are the socket's recv and send without flags (soo_read/soo_write).

• GuestSockets::Read and GuestSockets::Write in the socket runtime forward to recv and send with flags 0, so the socket layer's errno applies: EAGAIN (35) on an empty non-blocking socket, EPIPE (32) after the peer closed, EBADF (9) on a closed descriptor. A zero-length read returns 0 without touching the socket, as dofileread does, and a read longer than INT_MAX is clamped.
• read, write, _read and _write route descriptors at or above GuestSockets::FirstDescriptor to them; sceKernelRead and sceKernelWrite do the same, before their null-buffer check, and return the kernel error (0x80020023 for EAGAIN, 0x80020009 for EBADF) instead of throwing.
• Recorded in TechnicalDebt: EPIPE raises the host's SIGPIPE on Linux, which the guest can neither ignore nor suppress, and nothing on Windows (the console raises it unless SO_NOSIGPIPE); a write above INT_MAX bytes is EMSGSIZE; readv, writev, pread, pwrite, their sceKernel forms and AIO on a guest socket still go to the host.

The 15 catalog titles whose OpenSSL QUIC notifier creates a socketpair (boykopovar#1711, open) signal it with write and drain it with read (rio_notifier.c through writesocket/readsocket); the same applies to any TCP socket a title reads with read.

Overlap: boykopovar#1307 rewrites the four wrappers in Stdio.cpp (the socket branch has to stay first); boykopovar#1711 and boykopovar#1708 insert a TechnicalDebt line at the same place; boykopovar#1091 and boykopovar#1738 add declarations to SocketRuntime.hpp, and boykopovar#1091 code to Udp.cpp near the table, where this adds two declarations and two functions at the end. Whichever merges second rebases. The pr-overlap report's __error_nid_postfix rows (boykopovar#1613, boykopovar#1616, boykopovar#1627, boykopovar#1675) are the shared extern "C" declaration of the errno accessor that Open.cpp now adds, not an export.

Tested

• Linux (CachyOS, kernel 7.2.9, GCC 16.2.1), full build with BUILD_TESTING=ON and ANYPS5_ENABLE_SPIRV_TOOLS=ON on this branch (based on main b83e849): ctest --repeat until-fail:3 467/467; tools/check_test_registration.py and tools/check_conventions.py --base upstream/main pass.
• New guest_socket_read_write test over a TCP loopback pair: write/read, _write/_read and sceKernelWrite/sceKernelRead carrying data both ways; EAGAIN and 0x80020023 on an empty non-blocking socket; zero-length reads returning 0; 0 from both forms after the peer closed; (Linux) EPIPE on write after the peer closed, with the host's SIGPIPE ignored by the test since a title cannot; EBADF and 0x80020009 from every entry point on a closed descriptor. Each receive waits with poll first, accepting POLLHUP after the close. Written before the change: on main it aborts with the sceKernelWrite throw above; it passes with the change.
• Not tested on Windows; the routing is platform-neutral and the test excludes only the EPIPE case there, like guest_tcp.

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 in TechnicalDebt
• 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
• AI-assisted: yes (Claude Code)

🤖 Generated with Claude Code