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/object-store.json

@@ -28,7 +28,7 @@
     {"id": "grants", "type": "directory", "path": "/var/lib/objectstore/grants", "mode": "0700"},
 
     {"id": "store", "type": "container", "name": "mesh-store",
-     "image": "minio/minio@sha256:0000000000000000000000000000000000000000000000000000000000000000",
+     "image": "minio/minio@sha256:aefec8a86702aff0b0dcfdd9284bd7ab7c5631cbf9be63275799e6edcb30dfa2",
      "args": ["server", "/data"],
      "env": {"MINIO_ROOT_USER": "meshroot"},
      "ports": ["9000:9000"],