feat(cluster): self-first seed ordering closes the boot-alone outage gap

Every node now lists ITSELF as seed-nodes[0] and its partner second. Akka runs
FirstSeedNodeProcess -- the only bootstrap path that can form a NEW cluster when
no peer answers InitJoin -- exclusively for seed-nodes[0]; every other node runs
JoinSeedNodeProcess and retries InitJoin forever. That is why a lone cold-starting
central-b never came Up (the "registered outage gap"), and self-first ordering
closes it using Akka's own protocol.

- 6 node appsettings swapped (the *-node-b configs; the -a nodes were already
  self-first). All 14 shipped node configs now satisfy the invariant.
- StartupValidator enforces it at boot, comparing host AND port -- the invariant
  fails silently when broken, so it is enforced loudly. NOTE: the gitignored
  deploy/wonder-app-vd03/ overlay must be reordered before its next deploy or
  that node will refuse to boot.
- SelfFirstSeedBootstrapTests: real in-process clusters at production
  failure-detection timings, incl. a falsifiability control proving the OLD
  peer-first ordering never forms.

Rejected alternative (implemented, measured, discarded): an external self-form
timer calling Cluster.Join(SelfAddress) after a window. It sits outside Akka's
join handshake and so cannot tell "no seed answered" from "a seed answered and
the join is in flight". On a routine standby restart the peer is alive but the
join stalls behind removal of the node's own stale incarnation; a Join(self)
during TryingToJoin abandons the in-flight join and forms a second cluster at
the same address -- still split after 90s. Docs that claimed self-first ordering
was unsafe for simultaneous cold start are corrected: while mutually reachable
the InitJoin handshake converges them to one cluster (measured).

docker-env2/central-node-b/appsettings.Central.json

@@ -8,8 +8,8 @@
     },
     "Cluster": {
       "SeedNodes": [
-        "akka.tcp://scadabridge@scadabridge-env2-central-a:8081",
-        "akka.tcp://scadabridge@scadabridge-env2-central-b:8081"
+        "akka.tcp://scadabridge@scadabridge-env2-central-b:8081",
+        "akka.tcp://scadabridge@scadabridge-env2-central-a:8081"
       ],
       "SplitBrainResolverStrategy": "auto-down",
       "StableAfter": "00:00:15",