Fix child lookups, cache keys and loadstring

`loadstring` was removed in Lua 5.2, `load` takes a string the same way.

Warp gui child lookups are optional, so they use the same assert the
rest of the repo does.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

exp_legacy/module/expcore/roles.lua

@@ -894,7 +894,7 @@ role:set_permission_group('Admin')
 function Roles._prototype:set_permission_group(name, use_factorio_api)
     ExpUtil.assert_not_runtime()
     if use_factorio_api then
-        self.permission_group = { true, name }
+        self.permission_group = { true, name } --[[@as [boolean, string] ]]
     else
         local group = Groups.get_group_by_name(name)
         if not group then return end