grpc: descriptor-set based compatibility checking

[webdevsamran]

Context

Proto loading covers service/RPC removal, field-number reuse, wire-type risks, enum changes.

Work items

• Accept compiled FileDescriptorSet input (--descriptor-set) alongside .proto sources
• Message field presence semantics (proto3 optional vs oneof) in diff rules
• Streaming RPC direction changes (unary -> bidi) as breaking
• Reserved-range overlap detection

Note

Do not claim complete protobuf compatibility until each rule is covered by tests.