clanService/phonebox-global: decentralized phone network over yggdrasil

New service alongside phonebox. Boxes get a random 6 digit number
(100000-999999) signed by the machine's yggdrasil key. A serf gossip
cluster on yggdrasil port 7946 exchanges the signed records;
phonebox-sync verifies key -> address derivation and the signature and
writes the number -> address map to /run/phonebox/numbers for the
asterisk dialplan. Any yggdrasil node running the service can reach any
other; clan-core's ygg-input filter is opened for SIP, RTP and gossip.

Includes a two-node VM test proving directory convergence and a call
over yggdrasil. The existing phonebox service is untouched.

modules/clan/phonebox-global/flake-module.nix

@@ -0,0 +1,23 @@
+{
+  inputs,
+  self,
+  ...
+}:
+let
+  module = ./default.nix;
+in
+{
+  clan.modules = {
+    phonebox-global = module;
+  };
+  perSystem =
+    { ... }:
+    {
+      clan.nixosTests.service-phonebox-global = {
+        imports = [ ./tests/vm/default.nix ];
+        _module.args = { inherit self inputs; };
+
+        clan.modules."@clan/phonebox-global" = module;
+      };
+    };
+}