ssh-hostcert + enrol-broker: fix defects surfaced by the F-02 bench

F-02/P-17 SSH host-cert flow was proven end-to-end on comet-beacon-54
(2026-07-11); these fix the correctness/scoping/cleanup defects the bench
surfaced:

- ssh-hostcert: derive the FQDN principal <hostname>.<domain> from the DHCP
  search domain at runtime (resolv.conf), alongside the short hostname, so an
  admin `@cert-authority *.<domain>` line accepts by-FQDN logins instead of
  "name is not a listed principal". Never baked (invariant #1). + tests.
- ssh-ca-bootstrap.sh: export the SSH host CA public key in OpenSSH wire format
  (ssh-keygen -i -m PKCS8), not the PEM/PKCS8 that `step crypto keypair` writes
  -- a PEM @cert-authority anchor is unparseable and silently falls back to TOFU.
- ssh-ca-bootstrap.sh: fail loudly (and verify enableSSHCA=true) if the
  provisioner SSH-enable step fails, instead of swallowing it and shipping a CA
  that 502s every runtime sign.
- dot1x-bench: point the co-located broker at https://localhost:8443, not the
  fleet dnsmasq name ca.fleet.lab (unresolvable from the broker's host netns ->
  "no such host" at sign time). localhost is in the CA cert SAN.
- serving: remove the mock-signer enrol-broker copy -- two host-networked
  brokers collide on 9090/9443 and the mock issued untrusted certs. The real
  broker lives with step-ca (dot1x-bench); nginx /enrol/ still proxies to it on
  loopback.
- enrol-broker: retire the HTML admin console; the fleet-manager passkey SPA now
  drives the mTLS JSON API (/pending, /approve|deny). Removes a weaker,
  client-cert-only parallel human-approve path.

Design-doc bench note deferred: docs/ssh-host-certificate-design.md lives on the
secure-ntp branch (fbb8100), not here -- to be added there / on reconcile.

serving/dot1x-bench/docker-compose.yml

@@ -70,7 +70,12 @@ services:
       BROKER_PUBLIC_ADDR: "127.0.0.1:9090"
       BROKER_ADMIN_ADDR: "127.0.0.1:9443"
       BROKER_SIGNER: "step"
-      STEP_CA_URL: "https://${CA_DNS:-ca.fleet.lab}:${CA_PORT:-8443}"
+      # Co-located with step-ca (both network_mode: host), so reach it over
+      # loopback — NOT the fleet DNS name. ca.fleet.lab is a dnsmasq name for
+      # boot-LAN clients; the broker's host resolver doesn't know it, so signing
+      # died with "lookup ca.fleet.lab: no such host" (bench 2026-07-11). localhost
+      # is always in the CA cert SAN (DOCKER_STEPCA_INIT_DNS_NAMES) so TLS verifies.
+      STEP_CA_URL: "https://localhost:${CA_PORT:-8443}"
       STEP_CA_ROOT: "/pki/root_ca.crt"
       STEP_PROVISIONER: "enrol"
       STEP_PROVISIONER_PASSWORD_FILE: "/pki/stepca/secrets/password"