Maintenance / optimizations:
CL/683955: encoding/protowire: micro-optimize SizeVarint (-20% on Intel)
CL/674055: internal/impl: remove unnecessary atomic access for non-lazy lists
CL/674015: impl: remove unnecessary nil check from presence.Present
CL/673495: types/descriptorpb: regenerate using latest protobuf v31 release
CL/670516: cmd/protoc-gen-go: centralize presence and lazy logic into filedesc
CL/670515: internal: move usePresenceForField to internal/filedesc
CL/670275: internal/impl: clean up usePresenceForField() (no-op)