Implement Fiber DA spec compatibility (FiberFetcher)

[tac0turtle]

Summary

Implement FiberFetcher as a DataFetcher backend that conforms to the Fiber DA spec. This enables apex to fetch namespace data directly from the Fiber network instead of going through celestia-node, reducing latency and removing the celestia-node dependency for production deployments.

Context

The architecture already defines a pluggable DataFetcher interface (pkg/fetch/fetcher.go). The FiberFetcher implementation (pkg/fetch/fiber.go) is planned for Phase 4. This issue tracks aligning that implementation with the upstream Fiber DA spec as it stabilizes.

Requirements

• Implement FiberFetcher satisfying the DataFetcher interface:
  • GetHeader(ctx, height)
  • GetBlobs(ctx, height, namespace)
  • GetNetworkHead(ctx)
  • SubscribeHeaders(ctx)
  • Close()
• Conform to the data types and wire format defined in fibre-da-spec
• Add config support for type = "fiber" in [data_source]
• Integration tests against a local Fiber node (or mock)

Blocked on

• Fiber DA spec reaching a stable/tagged version
• Fiber network availability for testing

References

• Spec: https://github.com/celestiaorg/fibre-da-spec
• Design doc: test_plan.md — Phase 4, DataFetcher interface

ref celestiaorg/celestia-node#4777