fix(blackbox): unit had no Image and no Network=host - quadlet refused to generate it

The deploy died at 'Unit blackbox-exporter.service not found': a
.container without Image= is not generated at all, and the header
argued for Network=host it never declared. Completes the unit (digest
pinned prom/blackbox-exporter, Network=host) and fixes the bind
topology the first cut got wrong:

- Prometheus is bridge-networked and the netavark hairpin gap bars it
  from its own host's LAN IP, so a LAN-only prober bind is unscrapable
  and a loopback-only bind was claimed to ride the gateway on faith.
  The prober now binds 9115 on the podman gateway (scrape path) AND
  the LAN (healthcheck/diagnostics); the listen_address line leaves
  blackbox.yml so config and Exec cannot disagree.
- comfyui probe target http://comfyui:8188 -> http://127.0.0.1:8188:
  bridge container names do not resolve in the host netns; the
  published 0.0.0.0:8188 answers on loopback there.
- New seam: every observability unit must declare Image= (red on the
  shipped file by name); copier test pinned module presence instead
  of the removed listen line. Docs updated to the real topology.

shared/blackbox/blackbox.yml

@@ -3,11 +3,10 @@
 # 200 that fails the scrape protocol), which is why the old `services` and
 # `comfyui` jobs sat permanently red on `unsupported Content-Type`.
 # See serverAppConfigs-docs/Architecture/Stale Signal Audit.md §1.
-# Loopback bind: the image's default 0.0.0.0:9115 would ride the LAN on this
-# Network=host container. Prometheus (bridge) still reaches it — its traffic
-# arrives on the podman gateway interface, which is not loopback.
-network:
-  listen_address: 127.0.0.1:9115
+# The listen address is NOT set here: the quadlet unit's Exec passes
+# --listen-address (LAN bind — see the unit header for why loopback is
+# unreachable from the bridge-networked Prometheus). Keep this file to
+# modules only, so the two sources cannot disagree.
 
 modules:
   http_2xx: