Metatable impl

Indexer implementation comparison.

📄️Il2Cpp Dispatch* Indexer

Source: mt/TypeRegistryCommon.cpp, mt/MetaBinding.cpp

📄️Mono Three-Table Indexer Implementation

Source: Runtime/Mono/Mt/TypeMemberLuaIndexer.cs

📄️Implementation — Metatable (`impl/metatable/`)

This directory describes member indexing native/managed landing points on Il2Cpp and Mono. Lua-visible semantics follow ../../spec/metatable/02-INDEX.md.