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-connectivity/openssh/openssh_%.bbappend

@@ -12,6 +12,14 @@
 # on (/run/forgefirm/ssh-enabled, tmpfs, gone at reboot) or on the dev
 # image (/etc/forgefirm-dev). The guard sits in check_for_no_start, which
 # start, reload and restart call; stop is never gated.
+#
+# The host keys live under /data/forgefirm/ssh: sshd_check_keys makes
+# them at the first start (it reads the HostKey paths from the config),
+# and they stay across updates, so the machine's fingerprint does not
+# change with a release. The rootfs is read-only, and the
+# read-only-rootfs image feature, finding no key in the image, selects
+# sshd_config_readonly at rootfs time; both configs carry the same
+# HostKey lines, so that selection changes nothing.
 
 do_install:append() {
     for config in sshd_config sshd_config_readonly; do
@@ -21,10 +29,15 @@ do_install:append() {
             -e 's/^[#[:space:]]*PermitRootLogin .*/PermitRootLogin no/' \
             -e 's/^[#[:space:]]*PermitEmptyPasswords .*/PermitEmptyPasswords no/' \
             -e 's/^[#[:space:]]*PasswordAuthentication .*/PasswordAuthentication yes/' \
+            -e '/^[#[:space:]]*HostKey /d' \
             "$f"
+        for t in rsa ecdsa ed25519; do
+            echo "HostKey /data/forgefirm/ssh/ssh_host_${t}_key" >> "$f"
+        done
         grep -q '^PermitRootLogin no$' "$f" \
             && grep -q '^PermitEmptyPasswords no$' "$f" \
             && grep -q '^PasswordAuthentication yes$' "$f" \
+            && [ "$(grep -c '^HostKey /data/forgefirm/ssh/' "$f")" = 3 ] \
             || bbfatal "$config: the ForgeFIRM policy lines did not land"
     done