clanService/phonebox-global: unit tests for the directory sync logic

Add a flake check `phonebox-global-sync` that exercises phonebox-sync.py:
yggdrasil address derivation (fixed vector plus cross-check against the
yggdrasil binary), record verification (foreign address, bad or forged
signature, malformed tags, number format), collision tie-breaking, and
the files written for the dialplan including stale entry removal.

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

@@ -11,7 +11,7 @@ in
     phonebox-global = module;
   };
   perSystem =
-    { ... }:
+    { pkgs, ... }:
     {
       clan.nixosTests.service-phonebox-global = {
         imports = [ ./tests/vm/default.nix ];
@@ -19,5 +19,21 @@ in
 
         clan.modules."@clan/phonebox-global" = module;
       };
+
+      # Unit tests for the directory sync logic (record verification,
+      # address derivation, collision handling, file output).
+      checks.phonebox-global-sync =
+        pkgs.runCommand "phonebox-global-sync-test"
+          {
+            nativeBuildInputs = [
+              (pkgs.python3.withPackages (ps: [ ps.cryptography ]))
+              pkgs.yggdrasil
+            ];
+            PHONEBOX_SYNC = ./phonebox-sync.py;
+          }
+          ''
+            python3 ${./tests/sync/test_sync.py} -v
+            touch $out
+          '';
     };
 }