CelestiaNodeFetcher: upstream RPC client

[tac0turtle]

Summary

Implement CelestiaNodeFetcher (pkg/fetch/celestia_node.go) — the DataFetcher backend that connects to a running celestia-node via JSON-RPC over WebSocket.

Requirements

• Satisfy the DataFetcher interface:
  • GetHeader(ctx, height) — call header.GetByHeight
  • GetBlobs(ctx, height, namespace) — call blob.GetAll
  • GetNetworkHead(ctx) — call header.NetworkHead
  • SubscribeHeaders(ctx) — call header.Subscribe, return channel
  • Close() — clean shutdown of WebSocket connection
• Use celestia-openrpc as the client library (lightweight, no cosmos-sdk dependency)
• Auth token support for celestia-node's JWT authentication
• Automatic reconnection on WebSocket disconnect with exponential backoff
• Connection health check (periodic ping or detect stale subscription)

Configuration

[data_source]
type = "celestia-node"
endpoint = "ws://localhost:26658"
auth_token = ""

References

• Design doc: test_plan.md — Data Source Interface section
• celestia-openrpc client library
• celestia-node api/rpc/server.go — server-side RPC setup