kernel: processes have an environment now, and zsh is the shell

Porting zsh found four defects in the kernel and one in the filesystem. None
of them were about zsh; all of them had been there the whole time.

**exec threw the environment away.** kernel/fs/exec.c wrote a single zero for
envp with the comment "(no environment)" -- not yet, but ever, for every
process. musl has been passing a perfectly good envp to execve (Linux 59, which
SchOS answers) and it was never read. So $TERM was unset, $PATH was whatever
each program invented, and a variable exported in a shell did not survive the
exec meant to carry it. exec() takes an envp, sys_exec reads argument three,
and schinit manifests grew an Environment key. The userland two-argument
exec(path, argv) stays as a wrapper that passes null explicitly -- the raw stub
is execve now, so a short caller cannot leave %rdx holding a stray pointer.

**Which overflowed the kernel stack.** Three pointer arrays, ~1.8 KB, on a
4096-byte stack, under exec -> loaduvm -> readi -> SchFS -> btree. Pid 1 went
down on the first exec. The canary caught it and named the process, which is
what it is for. They live in one kalloc'd page now.

**waitpid returned a bare -1** for both "no children" and "interrupted". musl
reads [-4095,-1] as -errno, so userspace saw EPERM; zsh printed "wait failed:
operation not permitted" and abandoned the rest of /etc/zsh/zshrc. -ECHILD and
-EINTR now. Third time a bare -1 has cost real time -- on a system with no
permission model, EPERM should be read as "a syscall returned -1".

**NVMA was 64.** musl's allocator takes a mapping per size class and does not
release them eagerly, so compinit ran out of slots and got -1, which is EPERM,
which malloc turns into NULL, which zsh reports as "fatal error: out of memory"
-- on a machine with 1000 MB free. 256 slots, and -ENOMEM.

**And a small write to SchFS costs 58 ms.** 200 writes to one open file take
11.75 s; the same to /dev/null take 0.14 s. That is the design stated at the
top of kmount.c -- one transaction group per operation, and a commit is two
flushes and an uberblock -- not a bug. But compdump writes its digest with
thousands of separate prints, so it takes minutes and looks like a hang.
compinit -D, which skips the dump, finishes in 3.6 s. This one is a decision
rather than a fix, so it is written up and left: /etc/zsh/zshrc keeps compinit
switched off and explains exactly why, with the measurements.

zsh runs as /bin/zsh and is the console shell. ZLE works -- arrow keys recall
history through terminfo, $COLUMNS reads the real 160 because TIOCGWINSZ was
fixed, colour and prompt expansion work, startup is 0.14 s. mksh keeps /bin/sh,
so schinit's no-jobs fallback is a different program from the one that could
have failed.

Also fixed while here: AT_CLKTCK was the literal 100 and is USER_HZ; the
extract rule in four ports could move a source tree inside itself when make
re-ran it, which is how zsh's configure came to write modfile=zsh-5.9/... into
config.modules and fail three steps later.

schfs-check passes. Full writeup in docs/history/kernel.md section 14.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2

etc/zsh/zshrc

@@ -0,0 +1,113 @@
+# /etc/zsh/zshrc — the system-wide interactive zsh configuration for SchOS.
+#
+# Deliberately short. This sets up the things an interactive shell is useless
+# without -- history, completion, and the keys a console sends -- and stops.
+# Anything beyond that belongs in a user's own ~/.zshrc, which is the file zsh
+# reads after this one.
+
+# ---------------------------------------------------------------------------
+# History
+# ---------------------------------------------------------------------------
+HISTFILE=${HOME:-/root}/.zsh_history
+HISTSIZE=2000
+SAVEHIST=2000
+setopt APPEND_HISTORY INC_APPEND_HISTORY SHARE_HISTORY
+setopt HIST_IGNORE_DUPS HIST_IGNORE_SPACE HIST_REDUCE_BLANKS
+
+# ---------------------------------------------------------------------------
+# Behaviour
+# ---------------------------------------------------------------------------
+setopt AUTO_CD                 # `cd /tmp` is implied by typing `/tmp`
+setopt EXTENDED_GLOB
+setopt INTERACTIVE_COMMENTS    # a # in an interactive line is a comment
+setopt NO_BEEP                 # SchOS has no audio, but it would also scroll
+unsetopt FLOW_CONTROL          # ^S/^Q are worth more as keys than as flow control
+
+# ---------------------------------------------------------------------------
+# Keys
+# ---------------------------------------------------------------------------
+# Emacs bindings, because that is what every other line editor on the system
+# does -- mksh's default and the kernel's own canonical-mode editing both.
+bindkey -e
+
+# Home, End, Delete and the arrows, taken from terminfo rather than written
+# out as escape sequences. The sequences differ between the SchOS console and
+# a serial terminal; the terminal's own description does not.
+typeset -A _sch_keys=(
+  "${terminfo[khome]}"  beginning-of-line
+  "${terminfo[kend]}"   end-of-line
+  "${terminfo[kdch1]}"  delete-char
+  "${terminfo[kich1]}"  overwrite-mode
+  "${terminfo[kcuu1]}"  up-line-or-search
+  "${terminfo[kcud1]}"  down-line-or-search
+  "${terminfo[kcub1]}"  backward-char
+  "${terminfo[kcuf1]}"  forward-char
+)
+for _k _w in "${(@kv)_sch_keys}"; do
+  [[ -n $_k ]] && bindkey -- "$_k" "$_w"
+done
+unset _sch_keys _k _w
+
+# A terminal that has just been resized, or that zsh has just taken over, may
+# not have told terminfo yet. This puts the keypad in the mode the escape
+# sequences above were described for, and takes it back out on exit.
+if (( ${+terminfo[smkx]} && ${+terminfo[rmkx]} )); then
+  autoload -Uz add-zle-hook-widget 2>/dev/null
+  function _sch_zle_init { echoti smkx }
+  function _sch_zle_line_finish { echoti rmkx }
+  zle -N zle-line-init _sch_zle_init
+  zle -N zle-line-finish _sch_zle_line_finish
+fi
+
+# ---------------------------------------------------------------------------
+# Completion
+# ---------------------------------------------------------------------------
+# Completion is NOT initialised here yet, and the reason is a filesystem
+# measurement rather than anything to do with zsh.
+#
+# A small write to SchFS costs about 58 ms, because every operation is its own
+# transaction group and a commit is two cache flushes and an uberblock write --
+# see the comment at the top of kernel/fs/schfs/kmount.c, which describes the
+# trade-off exactly. Measured on this machine, 200 writes:
+#
+#     to one open file on SchFS   11.75 s   (58 ms each)
+#     open + write + close each   21.39 s   (107 ms each)
+#     to /dev/null                 0.14 s   (0.7 ms each)
+#
+# compdump writes the digest with a few thousand separate `print` calls, so
+# compinit -d takes minutes and looks exactly like a hang. compinit itself is
+# fine: with -D, which skips the dump, it finishes in 3.6 seconds.
+#
+# So this waits for deferred commits. When small writes stop costing a disk
+# barrier each, delete this comment and uncomment the three lines below.
+#
+#   autoload -Uz compinit
+#   compinit -u -d ${HOME:-/root}/.zcompdump
+#
+# Until then, completion is available on demand in a shell you do not mind
+# waiting three seconds for:
+#
+#   autoload -Uz compinit && compinit -D -u
+#
+# -u is not optional whenever this is turned back on. Without it compinit runs
+# compaudit, decides the function directories are "insecure" because SchOS has
+# no ownership model to check, and *prompts* for an answer on a shell that is
+# still starting up -- which is a real hang, unlike the one above.
+
+# These take effect as soon as compinit is turned on above; a zstyle with no
+# completion system to read it is inert, not an error.
+zstyle ':completion:*' menu select
+zstyle ':completion:*' matcher-list 'm:{a-z}={A-Za-z}'     # case-insensitive
+zstyle ':completion:*' list-colors ''
+zstyle ':completion:*:descriptions' format '%F{244}%d%f'
+
+# ---------------------------------------------------------------------------
+# Prompt
+# ---------------------------------------------------------------------------
+# The accent violet from include/theme.h, which is 99 in the 256-colour cube
+# and falls back to magenta on the 8-colour console. The last-status marker is
+# the only thing here that ever turns red -- same rule as the rest of the
+# system: normal states are calm, only failure is loud.
+setopt PROMPT_SUBST
+PROMPT='%F{magenta}%B%n@schos%b%f %F{cyan}%~%f %(?.%F{green}❯.%F{red}❯)%f '
+RPROMPT=''