CLPR-4.1: gRPC Server and ClprEndpointService

## Story CLPR-4.1: gRPC Server and ClprEndpointService

**Parent Epic:** CLPR-4: Hiero Endpoint

### Objective

Register the `ClprEndpointService` gRPC service alongside existing HAPI services on the node's Netty gRPC server.

### Acceptance Criteria

- **AC-1:** Implements `rpc sync(ClprSyncPayload) returns (ClprSyncPayload)`.
- **AC-2:** Implements `rpc discoverEndpoints(DiscoverEndpointsRequest) returns (DiscoverEndpointsResponse)` for gossip-based peer discovery. Responses throttled per caller.
- **AC-3:** Registered alongside existing HAPI services using the same `ServiceRegistration` mechanism. Shares the node's existing gRPC port.
- **AC-4:** The gRPC service is always registered. The `clprEnabled` flag is checked dynamically at call time — when disabled, calls return `UNAVAILABLE`. This allows the council to enable/disable CLPR via config update without node restarts (important as a safety kill-switch).
- **AC-5:** TLS for transport encryption. Peer authentication is proof-based (not certificate-based against an on-chain roster). Any peer providing valid proofs is legitimate; peers sending invalid data are shunned.
- **AC-6:** Inbound rate limiting: if a Connection's inbound sync rate exceeds `max_syncs_per_sec`, respond with `RESOURCE_EXHAUSTED`.
- **AC-7:** gRPC max message size configured to accommodate `max_sync_bytes`.
- **AC-8:** Graceful shutdown: in-flight syncs complete before server stops.
- **AC-9:** Sync handler: pre-verify received payload via verifier, construct this node's outbound payload, return response, submit caller's payload as `submitBundle`.

### Dependencies

- Existing Hiero gRPC/Netty infrastructure
- CLPR-1.1 (`ClprSyncPayload` proto, `DiscoverEndpointsRequest/Response` protos)

### Risk

**Medium** — Integration with existing gRPC service registration. Discovery throttling.

Start by locating the existing Hiero gRPC/Netty server, HAPI service registrations, and the ServiceRegistration mechanism. Read the CLPR-1.1 proto definitions and trace how configuration, verification, rate limiting, TLS, and graceful shutdown are handled. Done means ClprEndpointService is always registered, honors the listed call-time checks, supports both RPCs, and satisfies the acceptance criteria.

Written by the indexing model from the issue text.