zsh: two more cache answers that claimed capabilities SchOS lacks

Checking the rest of ZSH_CACHE after the killesrch fix, on the principle that
one wrong answer in a hand-written block is rarely the only one. Two more, both
`yes`, both measurable on the running system in a line each.

**zsh_cv_sys_path_dev_fd=/dev/fd.** Neither /dev/fd nor /proc/self/fd exists --
/proc/self does not exist at all. So `<(cmd)` failed at the point of use with

    cat: /dev/fd/11: No such file or directory

which reads as a broken command rather than as a feature the system does not
have.

**zsh_cv_sys_fifo=yes.** mkfifo() does not fail here. It *succeeds*, and
produces a character device: mknod() turns S_IFIFO into a T_DEV and keeps the
major/minor, so `ls -l` shows `crw-rw-rw- 438, 0` where a `p` belongs, and
nothing written to it comes back out. Succeeding wrongly is what made `yes`
look defensible.

Both `no` now. `<(cmd)` says "doesn't look like your system supports FIFOs",
which names the missing thing instead of blaming cat, and `$( )` is unaffected.

The two gaps are recorded in docs/architecture/filesystem.md rather than left
implied by a build flag -- named pipes and a path to one's own descriptors are
both real features, and neither exists.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/architecture/filesystem.md

@@ -269,6 +269,16 @@ both need the `..` fixup and the ancestor-loop check that are not written.
   a deadlock rather than a wrong answer.
 - **`rename()` does not move directories**, and will not replace one. `EISDIR`
   either way.
+- **No named pipes.** `mkfifo()` does not fail — it *succeeds* and produces a
+  character device, because `mknod()` turns `S_IFIFO` into a `T_DEV` and keeps
+  the major/minor. `ls -l` shows `crw-rw-rw- 438, 0` where a `p` belongs, and
+  nothing written to it comes back out. Succeeding wrongly is worse than
+  refusing: the zsh port was configured with `zsh_cv_sys_fifo=yes` on the
+  strength of it, and process substitution failed at the point of use rather
+  than being reported as unavailable.
+- **No `/dev/fd`, and no `/proc/self`.** A process cannot reach its own open
+  descriptors by path. `<(cmd)` needs one or the other — zsh now knows it has
+  neither and says so.
 - **No `AF_UNIX` socket inodes.** Socket names live in a table in
   `kernel/net/unix.c`, so `bind()` creates nothing that can be `stat()`ed or
   `unlink()`ed.