CelestiaAppFetcher: ingest from celestia-app (consensus node)

[tac0turtle]

Summary

Implement CelestiaAppFetcher satisfying the DataFetcher interface. Connects to a celestia-app (consensus/full) node via CometBFT RPC or gRPC instead of a celestia-node (light/bridge).

Motivation

Enables indexing without running a DA node — useful for operators already running validators or full nodes. Removes the hard dependency on celestia-node for data ingestion.

Approach

• Query blocks via /block + /block_results, extract blobs from block data
• Header subscription via CometBFT WebSocket /subscribe
• Evaluate whether to use CometBFT RPC (HTTP) or gRPC (cosmos-sdk dependency tradeoff — may need the submit/ submodule pattern from Isolate cosmos-sdk dependency in a separate Go submodule #24)

Interface

Must satisfy DataFetcher:

• GetHeader(ctx, height) (*Header, error)
• GetBlobs(ctx, height, namespaces) ([]Blob, error)
• GetNetworkHead(ctx) (*Header, error)
• SubscribeHeaders(ctx) (<-chan *Header, error)
• Close() error

Open Questions

• CometBFT RPC (zero cosmos-sdk deps, HTTP only) vs gRPC (richer API, pulls cosmos-sdk)?
• If gRPC, isolate in a submodule per Isolate cosmos-sdk dependency in a separate Go submodule #24?
• Blob extraction: parse from block transactions (MsgPayForBlobs) or use a dedicated endpoint?