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-users/forgefirm-users.bb

@@ -1,8 +1,9 @@
-SUMMARY = "ForgeFIRM operator accounts: record replay and the root shell warning"
-DESCRIPTION = "Replays the account record (/data/forgefirm/users, written \
-by forgectrl) into the system account files at boot and on reload, \
-removes the local accounts the record does not name, and installs the \
-warning an interactive root shell prints."
+SUMMARY = "ForgeFIRM operator accounts: record render and the root shell warning"
+DESCRIPTION = "Renders the account record (/data/forgefirm/users, written \
+by forgectrl) into the system account files at boot and on reload. The \
+rootfs is read-only: the four files show tmpfs copies, bind-mounted, and \
+a render writes through them; the accounts the record does not name are \
+left out. Also installs the warning an interactive root shell prints."
 LICENSE = "MIT"
 LIC_FILES_CHKSUM = "file://${COMMON_LICENSE_DIR}/MIT;md5=0835ade698e0bcf8506ecda2f7b4f302"
 
@@ -16,12 +17,12 @@ S = "${WORKDIR}"
 inherit update-rc.d
 
 INITSCRIPT_NAME = "forgefirm-users"
-# 05: rcS has run (mountall mounted /data at S03) and sshd starts at
-# S09, so the accounts exist before the first login can arrive.
+# 05: rcS has run (mountall mounted /data and /run at S03) and sshd starts
+# at S09, so the accounts exist before the first login can arrive.
 INITSCRIPT_PARAMS = "start 05 2 3 4 5 ."
 
-# useradd, groupadd, usermod, userdel, groupdel
-RDEPENDS:${PN} += "shadow"
+# No shadow tools: the read-only-rootfs image feature drops the shadow
+# package from the image, and the script writes the account lines itself.
 
 do_install() {
     install -Dm 0755 ${WORKDIR}/forgefirm-users.init ${D}${sysconfdir}/init.d/forgefirm-users