Protobuf Schema Audit

Audit one .proto source for what protobuf rejects and what it will cost later. Reports an absent syntax line, which silently means proto2; a required field, which can never be removed; an enum with no zero value, so an unset field has nothing to read as; a zero value named after a real state; a duplicate number; a number inside the implementation-reserved range; a non-permitted map key; and a repeated field above the one-byte tag boundary while low numbers are free.