feat(php-shim): implement syscall-backed functions via the nix crate

Several shim functions were left as todo!() because the standard library
exposes no equivalent and no syscall crate was available. Adding nix
unblocks them:

- proc_open now wires descriptors beyond stderr, creating the pipe itself
  and installing the child end with dup2(2) from pre_exec
- proc_terminate and posix_kill deliver arbitrary signals via kill(2)
- get_current_user reports the owner of the running executable
- php_uname answers every mode from uname(2) instead of only "s" and "r"

It also closes gaps that were previously approximated:

- fstat stats the stdio streams and pipes rather than reporting failure
- touch stamps mtime/atime on an existing path, including directories
- is_writable/is_executable use access(2) instead of permission bits
- umask falls back to the read-modify-write umask(2) off Linux

The hand-written repr(C) structs and extern "C" declarations for getpwuid,
utime, statvfs, fcntl and select are replaced by their nix wrappers, which
in turn lets disk_free_space drop its Linux-only cfg.

cli_set_process_title, setproctitle and the pcntl_signal pair stay as
todo!(): the first two need access to the process's own argv block, and
the latter two depend on the signal-handling subsystem rather than on
sigaction(2) itself.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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