Mount the rootfs read-only on both images

The rootfs mounted read-write, so a slot ran with its own files open to
change, and the factory-slot mounts rode along on the release image.
Both images now carry the read-only-rootfs feature: the ro root line and
the rcS default, the volatile links made at rootfs time, a writable copy
of /var/lib at boot, a build failure for a post-install that needs the
machine, and the removal of shadow, base-passwd, update-rc.d and
update-alternatives.

What must last or change at run time is handled file by file:

- forgefirm-users renders the four account files from the record into
  /run/forgefirm/accounts and bind-mounts each copy over its /etc file
  (useradd and the rest are gone with shadow); a render writes through
  the mount, and the image's own files apply until the first render.
- forgefirm-banner bind-mounts a copy of /etc/issue and writes the
  address block through it.
- sshd keeps its host keys under /data/forgefirm/ssh, so the fingerprint
  survives updates; both sshd configs carry the same HostKey lines.
- forgefirm-logging passes logrotate a state file under /var/run
  (logrotate refuses to run without one).
- forgefirm-persist points the boot timestamp and the random seed at
  /data/forgefirm.

The dev image appends the /factory slot mounts, without nofail (busybox
mount hands it to the kernel, which rejects it). The rootfs command
entries lose their semicolons: on scarthgap the value is the task's
vardeps, split on whitespace, so "name;" left the function body out of
the signature and a changed body did not remake the rootfs; with the
bodies tracked, the dev image's DATETIME string needs a vardepsexclude.
release.sh gains the read-only gate (root ro, no /factory line,
ROOTFS_READ_ONLY=yes, host keys on /data). image.health checks the
mounts, the account binds, the banner bind, the host keys and the
dev-only /factory mounts.

Proven on the bench reference (dev image 20260909140901): / ro, /data
rw, /var/lib a tmpfs copy, the four account files and /etc/issue bound
from tmpfs, the host keys in /data/forgefirm/ssh, no "Read-only file
system" line in any log; forgectrl.auth and commission.account-login (a
temporary account rendered, logged in over HTTPS and removed again),
kernel.latch-locked-idle and motion.liveness-probe PASS; logrotate runs
with the volatile state. forgetest unit tests 335 OK; both images build
clean, and debugfs on the built rootfs shows every setting above.

meta-forgefirm/recipes-forgefirm/forgefirm-logging/files/forgefirm-logging.init

@@ -22,6 +22,12 @@ CONF=/etc/logrotate.conf
 PIDFILE=/var/run/forgefirm-logging.pid
 LEGACY=/data/forgefirm/legacy-logs
 
+# logrotate's state file (the last rotation of each file). The rootfs is
+# read-only, so its default under /var/lib is not used; the rules are
+# size-capped, so a state that starts fresh at every boot loses nothing.
+# logrotate refuses to run at all when it cannot create the file.
+STATE=/var/run/forgefirm-logrotate.status
+
 RULES=/data/forgefirm/rsyslog-forgefirm.conf
 
 # The daemons log to /dev/log with non-blocking datagrams and drop what
@@ -78,10 +84,10 @@ case "$1" in
     render
     sweep_legacy
     [ -x "$LOGROTATE" ] || exit 0
-    "$LOGROTATE" "$CONF" 2>/dev/null
+    "$LOGROTATE" -s "$STATE" "$CONF" 2>/dev/null
     ( while :; do
         sleep 3600
-        "$LOGROTATE" "$CONF" 2>/dev/null
+        "$LOGROTATE" -s "$STATE" "$CONF" 2>/dev/null
       done ) &
     echo $! > "$PIDFILE"
     ;;