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_scenario/module/gui/debug/model.lua

@@ -4,7 +4,7 @@ local ExpUtil = require("modules/exp_util")
 local concat = table.concat
 local inspect = table.inspect
 local pcall = pcall
-local loadstring = loadstring --- @diagnostic disable-line
+local loadstring = load
 local rawset = rawset
 
 local Public = {}