build: add scm-1 rockspec for the pb runtime Installs the 12 modules under pb.* from runtime/pb/ via builtin build type; lua >= 5.1 is the only declared dependency since every other import (bit, datetime, digest, ffi, fiber, json, protobuf, utf8) is a Tarantool builtin. Plugin stays out of scope — it's a Go binary built via the Justfile, not a luarocks artifact. Source URL points at the planned sourcecraft.dev home that matches go.mod's module path.