Implement Celestia node JSON-RPC compatibility layer

[tac0turtle]

Summary

Implement the JSON-RPC server and API modules that replicate the celestia-node RPC interface ev-node expects. This is the core API surface that makes apex a drop-in replacement for celestia-node.

Context

ev-node communicates with celestia-node over JSON-RPC (go-jsonrpc). Apex must expose the same interface so ev-node requires zero modifications. This covers Phase 2 of the implementation plan plus the stub modules from Phase 3.

Requirements

Blob module (pkg/api/blob.go)

• Get(height, namespace, commitment) — query SQLite by height+ns+commitment
• GetAll(height, namespaces[]) — query SQLite, filter to tracked namespaces
• Subscribe(namespace) — real-time streaming via subscription manager
• GetProof(height, namespace, commitment) — retrieve stored NMT proofs
• Included(height, namespace, proof, commitment) — verify against stored data
• GetCommitmentProof(...) — return "not supported" error (requires full EDS)
• Submit(...) — return "not supported" error (read-only service)

Header module (pkg/api/header.go)

• GetByHeight(height) — local store with upstream fallback
• Subscribe() — stream new headers
• LocalHead() — last synced height
• NetworkHead() — query upstream

Stub modules

• share — return "not supported" errors
• fraud — return "not supported" errors
• blobstream — return "not supported" errors

Server (pkg/api/server.go)

• JSON-RPC server using go-jsonrpc (matching celestia-node's pattern)
• Configurable listen address ([rpc] address)
• Register all modules above

Verification

• Point ev-node at apex instead of celestia-node — should work without changes
• Test subscription streaming under load
• Verify error responses match celestia-node format for unsupported methods

References

• celestia-node api/rpc/server.go — server pattern
• celestia-node nodebuilder/blob/blob.go — module interface
• celestia-node api/client/read_client.go — required modules
• Design doc: test_plan.md — API Compatibility section