Add LuaCheck for static analysis

[refaim]

Summary

Add LuaCheck to catch bugs and code quality issues via static analysis.

Motivation

LuaCheck can detect real bugs such as accidental globals, unused variables, and variable shadowing. Some of the recently fixed issues (closure bugs in hook loops, unsafe table modification during iteration) are the kind of problems that static analysis can catch early.

Plan

1. Add a .luacheckrc with:
   • Lua 5.1 standard (WoW Lua version)
   • WoW API globals allowlist (there are community-maintained lists for this, e.g. from wow-declarations or wowless)
   • WIM-specific globals
2. Fix or suppress any initial findings
3. Document how to run it locally