ADR 0056: selectable ACME issuer, step-ca init from operator root, route labels

Piece A + B of ADR 0056, completing the internal-CA work in ff01ada.

Selectable issuer. acme-ca is now a role two providers can satisfy: step-ca
(internal CA) or the new public-acme (a fact-only module, no container/listen)
that serves Let's Encrypt production. A mesh assigns one or the other to satisfy
route-proxy's `requires: acme-ca`.

One directory shape for both. A provider serves the ACME directory's parts the
way the mesh already models any reachable service -- an address (`at`), a `port`
and a `path` -- and route-proxy composes `https://<at>:<port><path>`. step-ca
lets the mesh fill `at` (its node) and `port` (its single listen) and serves only
`path`; public-acme, not being a mesh service, serves all three (overriding `at`
with the public host). Same composition either way.

Empty root means the system trust store. Both providers serve `root`: step-ca
the operator root PEM (settled per mesh), public-acme an empty string. route-proxy
writes it to the CA bundle file unconditionally; the binary now reads an empty
bundle as "the root is already trusted by the OS" and falls back to system roots
(examples/route-proxy/main.go, committed on the mesh-control ADR-0056 branch).

step-ca inits from the operator's root. The operator's root cert, root key and
root-key password are mounted at the smallstep entrypoint's default init paths
(/run/secrets/root_ca.crt, root_ca_key, root_ca_key_password) with the matching
DOCKER_STEPCA_INIT_*_FILE vars, so `step ca init` adopts the operator's root
instead of self-generating one -- the CA that signs is the CA route-proxy trusts.

Route names are labels, not FQDNs. Every routed module now contributes a `label`
(the leftmost subdomain) instead of a full public hostname; the node's public
domain composes the name. Apex (novox.be) is left as a full name -- composeName
has no empty-label/apex convention yet (mesh-control follow-up).

Claude-Session: https://claude.ai/code/session_01LrgweAeERJYBg88c5cKDzF

modules/step-ca/module.json

@@ -11,7 +11,10 @@
     }
   ],
   "serves": {
-    "acme-ca": {}
+    "acme-ca": {
+      "path": "/acme/acme/directory",
+      "root": ""
+    }
   },
   "listens": [
     {
@@ -23,7 +26,9 @@
   ],
   "own-secrets": {
     "password": "/var/lib/mesh/step-ca/password",
-    "root-key": "/var/lib/mesh/step-ca/root-key"
+    "root-cert": "/var/lib/mesh/step-ca/root-cert",
+    "root-key": "/var/lib/mesh/step-ca/root-key",
+    "root-key-password": "/var/lib/mesh/step-ca/root-key-password"
   },
   "resources": [
     {
@@ -67,11 +72,16 @@
         "DOCKER_STEPCA_INIT_NAME": "Mesh Internal CA",
         "DOCKER_STEPCA_INIT_DNS_NAMES": "localhost,127.0.0.1",
         "DOCKER_STEPCA_INIT_ACME": "true",
-        "DOCKER_STEPCA_INIT_REMOTE_MANAGEMENT": "false"
+        "DOCKER_STEPCA_INIT_REMOTE_MANAGEMENT": "false",
+        "DOCKER_STEPCA_INIT_ROOT_FILE": "/run/secrets/root_ca.crt",
+        "DOCKER_STEPCA_INIT_KEY_FILE": "/run/secrets/root_ca_key",
+        "DOCKER_STEPCA_INIT_KEY_PASSWORD_FILE": "/run/secrets/root_ca_key_password"
       },
       "volumes": [
         "/var/lib/step-ca:/home/step",
-        "/var/lib/mesh/step-ca/root-key:/run/secrets/root-key:ro",
+        "/var/lib/mesh/step-ca/root-cert:/run/secrets/root_ca.crt:ro",
+        "/var/lib/mesh/step-ca/root-key:/run/secrets/root_ca_key:ro",
+        "/var/lib/mesh/step-ca/root-key-password:/run/secrets/root_ca_key_password:ro",
         "/var/lib/mesh/step-ca/config.json:/run/config/config.json:ro"
       ]
     }