Fix costume merging, restrict server names, add Centrix tag

- Mod merge: rebuild a merged bundle's chunk metadata the way the game
  writes it (one record per chunk, in chunk GUID order), so costumes
  from different mods built on the same game costume keep their
  textures and meshes
- Merge audit checks that layout, and there is a new unit test for it
- Server names: only letters, numbers, spaces and - _ [ ] ( ),
  1 to 64 characters; the server browser hides servers with other names
- New Centrix tag from the backend's "centrix" list: blue and white
  name, badge and item animation, ranked after Dev and before Creator

Server/server_config.cpp

@@ -230,7 +230,7 @@ std::optional<std::uint64_t> parse_scoring(std::string_view text) {
 }
 std::string config_error(const ServerConfig &c) {
     using namespace multiplayer;
-    if (c.name.empty() || c.name.size() > 64 || !valid_member_name(c.name)) return "name must be 1 to 64 characters.";
+    if (!valid_server_name(c.name)) return std::string("name must be ") + server_name_rule + ".";
     if (c.map.empty() || !valid_map_destination(map_destination(c.map)))
         return "map \"" + c.map + "\" is not a known map. Use a name like \"San Vansterdam\", or put the map's mod "
                "folder in Mods next to the server.";