feat: add Mixin, CreateFromMixins, and MergeTable utilities

Implement the WoW Mixin pattern used by modern addons:
- Mixin(obj, ...) — copies fields from mixin tables into obj
- CreateFromMixins(...) — creates a new table from mixin templates
- CreateAndInitFromMixin(mixin, ...) — creates and calls Init()
- MergeTable(dest, src) — shallow-merge src into dest

These enable OOP-style addon architecture used by LibSharedMedia,
WeakAuras, and many Ace3-based addons for class/object creation.

src/addons/lua_engine.cpp

@@ -5955,6 +5955,26 @@ void LuaEngine::registerCoreAPI() {
         "function tDeleteItem(tbl, item)\n"
         "    for i = #tbl, 1, -1 do if tbl[i] == item then table.remove(tbl, i) end end\n"
         "end\n"
+        // Mixin pattern — used by modern addons for OOP-style object creation
+        "function Mixin(obj, ...)\n"
+        "    for i = 1, select('#', ...) do\n"
+        "        local mixin = select(i, ...)\n"
+        "        for k, v in pairs(mixin) do obj[k] = v end\n"
+        "    end\n"
+        "    return obj\n"
+        "end\n"
+        "function CreateFromMixins(...)\n"
+        "    return Mixin({}, ...)\n"
+        "end\n"
+        "function CreateAndInitFromMixin(mixin, ...)\n"
+        "    local obj = CreateFromMixins(mixin)\n"
+        "    if obj.Init then obj:Init(...) end\n"
+        "    return obj\n"
+        "end\n"
+        "function MergeTable(dest, src)\n"
+        "    for k, v in pairs(src) do dest[k] = v end\n"
+        "    return dest\n"
+        "end\n"
         // String utilities (WoW globals that alias Lua string functions)
         "strupper = string.upper\n"
         "strlower = string.lower\n"