docs(debt): note how closed standard descriptors are reopened

docs/dev/debt/libc.md

@@ -39,6 +39,7 @@
 - Windows direct-memory backing reserves a paging-file section. A [direct memory page](../../../core/libs/prx/libc/include/WindowsMappings.hpp) mapped at one guest address is private write-watched memory at that address and is not committed in the section; a page mapped at a second address moves its whole resident run into the section (committed there) and every address becomes a view of it, tracked by write protection. An unmapped resident page that is not all zeros is kept in a private reservation until it is mapped again. Committed section contents remain across unmap/remap and are released when the backing and its remaining views are destroyed. A host import alias over part of a resident run is refused. Linux retains its existing backing. Peak gameplay memory and Windows versions other than the tested host remain unverified.
 - [getc_unlocked, putc_unlocked, getchar_unlocked, putchar_unlocked](../../../core/libs/prx/libc/src/StandardStreams.cpp) (libc) - byte I/O uses the host unlocked operations; guest EOF/error flags and macro counters follow `FileStream`. Linux status queries use `feof_unlocked`/`ferror_unlocked` because [glibc `feof`](https://github.com/bminor/glibc/blob/d2097651cc57834dbfcaa102ddfacae0d86cfb66/libio/feof.c) and [`ferror`](https://github.com/bminor/glibc/blob/d2097651cc57834dbfcaa102ddfacae0d86cfb66/libio/ferror.c) can acquire the stream lock. Windows uses [the CRT no-lock byte operations](https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/fgetc-nolock-fgetwc-nolock) and `feof`/`ferror`, verified under a held stream lock on UCRT. Console behavior and other host CRTs remain unverified.
 - [fgetws, getwc, getwchar, putwc, putwchar](../../../core/libs/prx/libc/src/FormattingWide.cpp) (libc) - wide stream I/O encodes and decodes wide characters as UTF-8 on the underlying byte stream; `fgetws` reads characters into the destination buffer until newline or EOF and null-terminates, returning null on stream error. Console wide stream buffering and encoding details remain unverified.
+- [Closed standard descriptors](../../../core/libs/prx/libc/src/StandardStreams.cpp) (libc) - on Linux and macOS, a descriptor 0, 1 or 2 that is closed when libc loads is opened on `/dev/null` (read-only for 0, write-only for 1 and 2) by a static initializer of libc, which runs before the libraries that depend on libc initialize and before the title starts. Within libc this relies on no static initializer opening a descriptor before it, which `guest_closed_standard_descriptors` checks. glibc does the same for set-user-ID programs ([`check_fds.c`](https://sourceware.org/git/?p=glibc.git;a=blob;f=csu/check_fds.c)). Without it the first descriptor a library opens, the write watch's `userfaultfd` for one, becomes the guest's standard input or output. What the console gives a title as its standard descriptors is not measured, and on Windows a process started without standard handles is not handled.
 - [ftrylockfile](../../../core/libs/prx/libc/src/RuntimeSupport.cpp) (libc) - tries the host CRT stream lock that `flockfile`, `funlockfile` and each locking host stdio call on the stream take, so it returns -1 while another thread holds the stream through `flockfile` or is inside such a call, and 0 when it acquires the lock or already owns it (recursively, released by as many `funlockfile` calls), as FreeBSD 11 [`_ftrylockfile`](https://github.com/freebsd/freebsd-src/blob/8c639945a44588f6cfdddfb057bb5aad88735da0/lib/libc/stdio/_flock_stub.c#L89-L104) does. A libc export holds the lock only for each host call it makes, not across several host calls or while it updates the guest `FILE` prefix. Linux uses glibc `ftrylockfile`. UCRT has no try variant of `_lock_file`, so Windows calls `TryEnterCriticalSection` on the `CRITICAL_SECTION` that `_lock_file` enters, found through the private UCRT stream layout (`__crt_stdio_stream_data` in `ucrt/inc/corecrt_internal_stdio.h`, unchanged from Windows SDK 10.0.19041 to 10.0.26100); another CRT or a change of that layout breaks it. A null or closed stream throws where FreeBSD dereferences it
 - [Wide-character conversions](../../../core/libs/prx/libc/src/Multibyte.cpp) (libc) - The wide `wcsto*` functions parse the ASCII prefix of the guest string, as the C locale accepts only ASCII white space and digits. [`wcsrtombs`](../../../core/libs/prx/libc/src/Multibyte.cpp) uses the guest's single-byte C locale through `wcrtomb`; other encodings and shift states are not supported
 - [realpath](../../../core/libs/prx/libc/src/RealPath.cpp) (libc) follows FreeBSD 11's [`realpath.c`](https://github.com/freebsd/freebsd-src/blob/stable/11/lib/libc/stdlib/realpath.c) except for symlinks. The console's guest filesystem has none, so a host symlink under the guest root is followed for existence and type but not expanded, and `..` drops the previous component lexically, as `Resolve` does for every guest path. Through a host link `linkdir -> sub/deep`, `linkdir/../x` gives `x` beside the link, not beside its target.