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.

docs/tpm-enrolment-design.md

@@ -348,8 +348,9 @@ exists. Enrolment is where it comes into being.
    `tpm-selftest.sh` capability check; step-ca + EAP-TLS bench.
 2. **Broker — BUILT (2026-06-28): [`serving/enrol-broker/`](../serving/enrol-broker/).**
    Go, stdlib-only. Public unauth listener (`POST /request`, `GET /status/{id}`)
-   behind the serving nginx `/enrol/`; admin **mTLS web UI** (`/`, `/pending`,
-   `/approve|deny/{id}`) on the mgmt network; wordlist fingerprint = `H(CSR-pubkey ‖
+   behind the serving nginx `/enrol/`; admin **mTLS JSON API** (`/pending`,
+   `/approve|deny/{id}`) on the mgmt network — driven by the fleet-manager passkey
+   SPA (the broker's own HTML console was retired 2026-07-11); wordlist fingerprint = `H(CSR-pubkey ‖
    server-nonce)`; time-boxed in-memory store + per-IP rate limit; signs via
    `step ca token`+`step ca sign` (broker holds the provisioner password) with a
    dev `mock` signer. **Fingerprint-only assurance.** Admin client CA is a