Add generic proto handler shell

[powerslider]

Summary

Adds the generic, reusable shell that per-RPC EVM sync handlers plug into. Relates to #5376.

Structure Outline

• Handler[Req, Resp]: typed p2p.Handler that owns proto
  marshal/unmarshal once and delegates per-RPC work to a Responder.
• Responder[Req, Resp]: the per-RPC contract. Return semantics:
  • (resp, nil) deliver resp to the peer
  • (zero, nil) drop (application-level reject)
  • (zero, err) server bug, surfaces as p2p.ErrUnexpected
• ProtoMessage: proto.Message + comparable, so a responder can
  return the zero value to signal "drop".