docs: spec out msgpack encoding and gRPC transport story

Two design slices captured under docs/specs/ for later pickup:

- msgpack_encoding.md: proto schemas as IDL, msgpack as wire format.
  Default mp_map keyed by field number; ARRAY/MP_TUPLE opt-ins for
  box.space feeders; WKT bridging to MP_DATETIME / MP_INTERVAL.
- grpc_transports.md: Connect-JSON over HTTP/1.1 as default external
  transport, net.box tunnel for in-cluster, explicit no on HTTP/2
  termination (recommend Envoy). Conformance anchored on
  connectrpc/conformance — same framed-runner shape as the existing
  protobuf conformance harness.

PLAN.md cross-links both specs from M4, Open Questions, and Non-goals.