observability: give Dankserver its node-exporter back as a Quadlet

Dankserver's node-exporter has been dead since a reboot: `podman ps -a` showed
it `Exited (143)` and Prometheus' `node-dankserver` job has been scraping
nothing. It ran only from `dankserver/compose.yaml`, which is the failure mode
this repository's boot-persistence rule exists for (rootless Podman does not
restart bare containers after the stop that shutdown performs), and the compose
form bound the wildcard address — the unauthenticated-metrics-API-on-every-
interface posture already removed on Dankclaw.

  - dankserver/observability/quadlet/node-exporter.container — mirrors the
    Dankclaw unit, listening on exactly 127.0.0.1:9100 (host-local probes) and
    192.168.0.235:9100 (Prometheus on Dankclaw), with Restart=always and
    WantedBy=default.target.
  - the compose service is deleted: one owner per container name.
  - deploy_observability now installs from "$TARGET/observability/quadlet"; it
    hardcoded the Dankclaw path, so a Dankserver unit could not be installed by
    the shared dispatcher at all.
  - planner: "node-exporter" joins the Dankserver service set, with a path rule
    for dankserver/observability/**; the dispatcher's allowlist gains
    dankserver:node-exporter.

Seams cover the silent halves: both listen addresses present and the wildcard
absent, the compose service gone, the allowlist entry present, and the planner
mapping the path to the service.

dankserver/compose.yaml

@@ -147,25 +147,8 @@ services:
       retries: 3
 
 
-  # ═══════════════════════════════════════════════
-  # Node Exporter — host metrics
-  # ═══════════════════════════════════════════════
-  node-exporter:
-    image: prom/node-exporter:latest
-    container_name: node-exporter
-    restart: unless-stopped
-    network_mode: host
-    pid: host
-    volumes:
-      - /proc:/host/proc:ro
-      - /sys:/host/sys:ro
-      - /:/rootfs:ro
-    command:
-      - --path.procfs=/host/proc
-      - --path.sysfs=/host/sys
-      - --path.rootfs=/rootfs
-    healthcheck:
-      test: ["CMD", "wget", "--spider", "-q", "http://localhost:9100/metrics"]
-      interval: 30s
-      timeout: 10s
-      retries: 3
+  # node-exporter left this file on 2026-09-16 for
+  # dankserver/observability/quadlet/node-exporter.container: a compose-only
+  # container does not survive a reboot here — it died `Exited (143)` and stayed
+  # dead, leaving Prometheus' node-dankserver job scraping nothing — and the
+  # compose form bound the wildcard address. One owner per container name.