fix(map): reject a zones layer without a deployment section

The README documents deployment as required when zones exist, but load_map
only enforced the reverse direction (deployment without zones). A map with
a zones layer and no deployment now fails with MapValidationError instead
of loading with unreachable deployment zones.

tests/test_map.py

@@ -111,6 +111,12 @@ def test_rejects_deployment_without_zone(tmp_path: Path) -> None:
         load_map(_write(tmp_path, body))
 
 
+def test_rejects_zones_without_deployment(tmp_path: Path) -> None:
+    body = _minimal_map() + "zones: |\n  ...\n  .A.\n  ...\n"
+    with pytest.raises(MapValidationError, match="no deployment section"):
+        load_map(_write(tmp_path, body))
+
+
 def test_rejects_shared_deployment_zone_letter(tmp_path: Path) -> None:
     """Two sides bound to the same zone letter would deploy on top of each
     other (build_states resets its placed set per side): a data error."""