feat(libkernel_sys): create symlink targets with parent-directory components

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/dev/debt/libkernel_sys.md

@@ -19,6 +19,7 @@
 - [mkdirat](../../../core/libs/prx/libkernel_sys/Export.cpp) (libkernel_sys) passes `AT_FDCWD` (-100) and absolute paths to `mkdir`. Any other relative path is joined to the descriptor's host directory, found the same way as for `fchdir`, so guest path aliases and case-insensitive name matching do not apply to it, and a `..` component is not kept inside the guest root. It returns `EBADF` for a descriptor that is not open and `ENOTDIR` for one that is not a directory. On Windows, `mode` is ignored. Console behaviour is unverified
 - [readlink](../../../core/libs/prx/libkernel_sys/Export.cpp) (libkernel_sys) on Windows reads a symbolic link's target from its reparse data and returns it with `/` separators, only when the target is relative, as `symlink` creates them. A symbolic link with an absolute target returns its print name, or its substitute name when the print name is empty, without a `\??\` or `\\?\` prefix. A directory junction returns the same kind of name when it is a drive path (`X:\...`), using the print name when that is a drive path and the substitute name otherwise. A mounted volume (`\??\Volume{GUID}\`), any other name surrogate and every reparse point without the name surrogate bit (WOF compression, deduplication, cloud files) return `EINVAL`, as FreeBSD does for a file that is not a link and as CPython's `os.readlink`, which reads only symbolic links and junctions, does
 - [aio_read, aio_write, aio_fsync, aio_error, aio_return, aio_suspend, aio_waitcomplete, aio_cancel](../../../core/libs/prx/libkernel_sys/Aio.cpp) (libkernel_sys) - a request with `SIGEV_SIGNAL`, `SIGEV_THREAD_ID` or `SIGEV_KEVENT` completion, or on a socket, throws, because delivering a guest signal or kevent and socket aio are not modelled. One worker thread runs the process's requests one at a time in submission order, so a request is cancellable only until that thread takes it. A queued request keeps the descriptor number, not the open file: closing or reusing the descriptor before it runs changes what it reaches. Pipes fail with `EOPNOTSUPP`, as FreeBSD 11 does with `vfs.aio.enable_unsafe` 0; the console's setting is unverified. Waits are not interrupted by signals (`EINTR`), and timeouts under 1 ms round up to 1 ms (hz 1000). Otherwise the rules follow FreeBSD 11 `vfs_aio.c`
+- [symlink](../../../core/libs/prx/libkernel_sys/Export.cpp) (libkernel_sys) stores the target unchanged in a host symbolic link, which the host follows later, not the guest path resolver. It throws for an absolute target, which would need a host path in the link and a translation back in `readlink`. A target with a `..` component is created only when the host and the guest would reach the same place: every directory a `..` leaves must exist and not be a symbolic link, so the host's physical `..` matches the guest resolver's lexical one, and the target joined to the link's host directory must equal the guest path it names, resolved through the guest mounts. A `..` that climbs out of a mount such as `/savedata0`, or past a directory reached through a symbolic link, throws, where FreeBSD 11.4's `namei` follows it from the link's directory. Console behaviour is unverified
 - [readlinkat](../../../core/libs/prx/libkernel_sys/Export.cpp) (libkernel_sys) passes `AT_FDCWD` (-100) and absolute paths to `readlink`. Any other relative path is joined to the descriptor's host directory, found the same way as for `fchdir`, so guest path aliases and case-insensitive name matching do not apply to it, and a `..` component is not kept inside the guest root. It returns `EBADF` for a descriptor that is not open and `ENOTDIR` for one that is not a directory. Console behaviour is unverified
 - [link](../../../core/libs/prx/libkernel_sys/Export.cpp) (libkernel_sys) on Windows resolves a symbolic link source through the host, which follows longer chains than FreeBSD: a chain of 40 links is linked, where FreeBSD 11.4 returns `ELOOP` after 32 (`MAXSYMLINKS`). A loop returns `ELOOP` as on FreeBSD
 - [fstatat](../../../core/libs/prx/libkernel_sys/Export.cpp) (libkernel_sys) passes `AT_FDCWD` (-100) and absolute paths to `stat`, or to `lstat` with `AT_SYMLINK_NOFOLLOW` (0x200). Any other relative path is joined to the descriptor's host directory, found the same way as for `fchdir`, so guest path aliases and case-insensitive name matching do not apply to it, and a `..` component is not kept inside the guest root. It returns `EBADF` for a descriptor that is not open and `ENOTDIR` for one that is not a directory, and flags other than `AT_SYMLINK_NOFOLLOW` return `EINVAL`. PPSA74126 imports it. Console behaviour is unverified