Initial commit: protoc-gen-tarantool plugin + pb runtime A protoc plugin (Go) and a pure-Lua + LuaJIT-FFI runtime that give Tarantool a complete proto3 + gRPC stack. Two codegen modes (full inline / runtime descriptor), 226-test luatest suite, 18-fixture mainline-protoc interop corpus, JSON codec, well-known types, gRPC client/server factories, runtime .proto parser, microbench harness with allocation regression gate. Covers PLAN.md M1-M5. Module is `pb` (not `protobuf`) to avoid colliding with Tarantool's built-in encode-only `protobuf` module.