fix(observability): serve every exporter bind; scrape same-host cadvisor via gateway

Two shipped-2026-10-20 bugs that left gpu-dankclaw, gpu-dankserver and
cadvisor-dankclaw DOWN beside a green deploy:

- gpu-exporter.py built one ThreadingHTTPServer per bind but ran
  serve_forever() on servers[0] only: the LAN-class socket sat listen()ing
  but never accept()ed, loopback deploy gate passed, every scrape that
  reached the other bind timed out on the SYN backlog. Now one thread per
  extra bind. Regression test spins --serve on two loopback binds and
  requires BOTH to answer /metrics.

- cadvisor-dankclaw scraped its own host's LAN IP. Measured from inside the
  Prometheus container: a bridge container cannot dial its own host's LAN
  address at all (hairpin refuses published ports too); the podman gateway
  answers. Target moved to host.containers.internal:8080; the seam that
  pinned the LAN address (asserting the wrong doctrine) is re-pinned to the
  gateway plus a negative pin against the own-host LAN dial. Observability
  page records the measured reachability law replacing the inferred one.

scripts/gpu-exporter.py

@@ -32,6 +32,7 @@ import glob
 import os
 import subprocess
 import sys
+import threading
 from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
 
 FAMILIES = [
@@ -198,6 +199,11 @@ def main(argv):
             return 2
         servers.append(ThreadingHTTPServer((b, args.port), Handler))
     sys.stderr.write("gpu-exporter serving :%d on %s\n" % (args.port, ",".join(binds)))
+    # EVERY socket must be served: a listen()ed-but-never-accept()ed bind looks
+    # healthy in `ss` yet times out every scrape once its SYN backlog fills —
+    # and the loopback deploy gate cannot see it. One thread per extra bind.
+    for s in servers[1:]:
+        threading.Thread(target=s.serve_forever, daemon=True).start()
     servers[0].serve_forever()
     return 0