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.

src/pf1e_simulator/map.py

@@ -131,6 +131,13 @@ def _check_deployment(
     deployment: dict[str, str], zones: tuple[str, ...] | None, path: Path
 ) -> None:
     if not deployment:
+        if zones is not None:
+            msg = (
+                "zones layer declared but no deployment section (the README "
+                "requires deployment when zones exist: each zone letter needs "
+                "a side)"
+            )
+            raise MapValidationError(path, msg)
         return
     if zones is None:
         msg = "deployment declared but no zones layer"