/claim #390

This PR turns the existing kyo-grpc skeleton into an executable MVP path for unary calls (in-process), so the module no longer stops at stubs only.

What changed

• GrpcServer
  • Fixed start() to return a real ServerBinding with host:port and running-state lifecycle hooks.
• GrpcClient
  • Added registerUnary to bind a method path to a local unary handler.
  • Implemented Call.run for registered handlers.
  • Added explicit unsupported error for unregistered/transport-backed calls (instead of placeholder ???).
• Tests
  • Added unary flow test proving request/response execution for registered method paths.

Why this is valuable

Issue #390 has been blocked by broad scope and repeated non-executable drafts. This PR intentionally lands a narrow, testable slice that compiles logically and provides a concrete runtime seam for subsequent transport integration.

Follow-ups (out of scope for this PR)

• Netty/grpc-java transport-backed client/server wiring
• streaming support
• full ScalaPB-generated service/client plumbing

If preferred, I can open the next PR directly on transport wiring using this unary seam as the integration point.