docs(3202): vhosts.nix's header described the file before the split

The header still announced "the forge, matrix and authelia sub-domain
vhosts" and an "Accept-header SPA map". None of the three is true:

- the four swarm vhosts moved into their own service modules, so the
  header named contents that are no longer in the file;
- `_` is described as serving the dashboard and per-agent routing, but
  it is now a bare 444 catch-all — that surface moved to the vhost
  named for the hive domain;
- the "Accept-header SPA map" is not merely gone, it is the mechanism
  the file's own dashboard comment argues *against* ("routing is by
  PATH, never content-type"), 140 lines below the header claiming it.

A file's opening comment is its orientation text — the first thing read
and the last thing checked — so a stale one costs more than a stale
line in the body. Rewritten to describe what the file holds now, and to
name the property the split established: a service owns its whole
gateway surface in its own module, and a service name appearing here is
the signal that the coupling is growing back.

nix/host-modules/hive-gateway/vhosts.nix

@@ -1,9 +1,17 @@
-# nginx virtual-host tree for the gateway: the `_` default server
-# (dashboard, per-agent routing, matrix discovery), the forge, matrix
-# and authelia sub-domain vhosts, and the Accept-header SPA map for the
-# matrix GUI. Pure function — called from ./default.nix with the
-# outer-scope config values as arguments; returns
-# `{ virtualHosts }`.
+# nginx virtual-host tree for the gateway: the `_` default server —
+# now only a catch-all that rejects — and the hive's own domain, which
+# carries the dashboard, per-agent routing, matrix discovery and the
+# swagger UI.
+#
+# ⚠️ The swarm services' vhosts are NOT here. Forge, matrix and authelia
+# each declare their own, built from the same kit this file consumes
+# (`services.hyperhive.gateway.lib`, ./vhost-lib.nix), so a service owns
+# its whole gateway surface — vhost and dns name — inside its own
+# module. A service name appearing in this file is the signal that the
+# coupling is growing back.
+#
+# Pure function — called from ./default.nix with the outer-scope config
+# values as arguments; returns `{ virtualHosts }`.
 {
   lib,
   cfg, # services.hyperhive.gateway