Pin the example modules to images that exist

novox/hq 04-ISSUES/025. Every image reference in every example module
was sixty-four zeros — eighteen of them across five modules. Each
parsed, resolved, and composed into a declaration a host accepts, and
none could ever have started: the machine reaches `docker pull` and
stops. That is why those modules were written and not running, and no
check saw it because every check passed.

The host validates the shape of a reference and nothing more, which is
correct: verifying a digest exists means reaching a registry, and that
is the one thing a host must never have to do. So the last place that
could catch this is the wrong place to try.

The guard therefore sits where a declaration is composed, not where a
manifest is parsed. A file in a repository is allowed to await a pin —
the design already says the manifest in a repository names artifacts
while the manifest the mesh holds names digests, and the bundle works
exactly that way. What must never happen is a placeholder reaching a
machine, and composing is the last moment before one does.

Twelve third-party images resolved to real digests without pulling
anything, which is also the mechanism the open issue needs. Two
discoveries came free: mailu publishes to ghcr rather than Docker Hub,
so seven references named repositories that do not exist at all; and it
renamed roundcube to webmail, so that one would have failed even with
the right registry.

What stays a placeholder is the mesh's own provisioner images, which
genuinely have no digest until built and pushed — the bundle's problem,
legitimately unresolved here. The stand-in consumer now stands in with
a real image rather than an invented one.

examples/modules/mailu.json

@@ -31,24 +31,24 @@
     {"id": "net", "type": "network", "name": "mailu"},
 
     {"id": "resolver", "type": "container", "name": "mailu-resolver",
-     "image": "mailu-unbound@sha256:0000000000000000000000000000000000000000000000000000000000000000",
+     "image": "ghcr.io/mailu/unbound@sha256:142aaad82ad1b0d5b59a5f1303778dba61a3e0a540f5d969c48862bcc99f6f5d",
      "network": "mailu",
      "env-file": ["/var/lib/mailu/secret.env"]},
 
     {"id": "redis", "type": "container", "name": "mailu-redis",
-     "image": "redis@sha256:0000000000000000000000000000000000000000000000000000000000000000",
+     "image": "redis@sha256:1db42ccef14898aa29bae778452d567534b59c107129cbc1163fb552de184d3c",
      "network": "mailu",
      "volumes": ["/services/mailu/data/redis:/data"]},
 
     {"id": "admindb", "type": "container", "name": "mailu-admindb",
-     "image": "postgres@sha256:0000000000000000000000000000000000000000000000000000000000000000",
+     "image": "postgres@sha256:7456ef82e5f5bc43d997f4781bbd7c0d6389bff397564649a356e206ba473aee",
      "network": "mailu",
      "env": {"PGDATA": "/var/lib/postgresql/data/pgdata"},
      "env-file": ["/var/lib/mailu/database.env"],
      "volumes": ["/services/mailu/data/data/psql_admindb/pgdata:/var/lib/postgresql/data/pgdata"]},
 
     {"id": "admin", "type": "container", "name": "mailu-admin",
-     "image": "mailu-admin@sha256:0000000000000000000000000000000000000000000000000000000000000000",
+     "image": "ghcr.io/mailu/admin@sha256:dcac20e9cbdad560faef9653b1b5ac0d9266f4098dc00f0e7f0d35f4e70ed8f1",
      "network": "mailu",
      "env-file": ["/var/lib/mailu/secret.env", "/var/lib/mailu/database.env", "/var/lib/mailu/admin.env"],
      "volumes": [
@@ -57,7 +57,7 @@
      ]},
 
     {"id": "imap", "type": "container", "name": "mailu-imap",
-     "image": "mailu-dovecot@sha256:0000000000000000000000000000000000000000000000000000000000000000",
+     "image": "ghcr.io/mailu/dovecot@sha256:46d18ba51032be8ebd6841aa49c1ef8762c729038c5fd86a081b5b884d478af9",
      "network": "mailu",
      "env-file": ["/var/lib/mailu/secret.env"],
      "volumes": [
@@ -66,25 +66,25 @@
      ]},
 
     {"id": "smtp", "type": "container", "name": "mailu-smtp",
-     "image": "mailu-postfix@sha256:0000000000000000000000000000000000000000000000000000000000000000",
+     "image": "ghcr.io/mailu/postfix@sha256:bbf882880f68849511710b35237a933f3fe80c4b28bf48ff20205dbd1f1433d7",
      "network": "mailu",
      "env-file": ["/var/lib/mailu/secret.env"],
      "volumes": ["/services/mailu/data/mailqueue:/queue"]},
 
     {"id": "antispam", "type": "container", "name": "mailu-antispam",
-     "image": "mailu-rspamd@sha256:0000000000000000000000000000000000000000000000000000000000000000",
+     "image": "ghcr.io/mailu/rspamd@sha256:e87ab93dd252cc69499caa5317dd10d445fd4291a7ecf6bca09793c7d475a0c8",
      "network": "mailu",
      "env-file": ["/var/lib/mailu/secret.env"],
      "volumes": ["/services/mailu/data/filter:/var/lib/rspamd"]},
 
     {"id": "webmail", "type": "container", "name": "mailu-webmail",
-     "image": "mailu-roundcube@sha256:0000000000000000000000000000000000000000000000000000000000000000",
+     "image": "ghcr.io/mailu/webmail@sha256:076b720fc766e58a97321cdb700e887c2008d6d323685fe59f323088333059dc",
      "network": "mailu",
      "env-file": ["/var/lib/mailu/secret.env"],
      "volumes": ["/services/mailu/data/webmail:/data"]},
 
     {"id": "front", "type": "container", "name": "mailu-front",
-     "image": "mailu-nginx@sha256:0000000000000000000000000000000000000000000000000000000000000000",
+     "image": "ghcr.io/mailu/nginx@sha256:09f28ab6d36367fcacc7994f7021f132ac845bdc05f04bf80906102d11aaa057",
      "network": "mailu",
      "env-file": ["/var/lib/mailu/secret.env"],
      "ports": ["25:25", "465:465", "587:587", "993:993", "7080:80"],