Sync engine: backfill, streaming, and gap recovery

[tac0turtle]

Summary

Implement the sync engine (pkg/sync/) that orchestrates fetching namespace data from upstream and persisting it to the store.

State machine

INITIALIZING → BACKFILLING → STREAMING
                    ↑            │
                    └────────────┘ (gap detected)

Components

Coordinator (pkg/sync/coordinator.go)

• Manages state transitions between backfill and streaming modes
• Loads checkpoint from store on startup
• Determines start height from config or last checkpoint

Backfill (pkg/sync/backfill.go)

• Fetches heights [start, network_head] in configurable batches (default: 100)
• Concurrent fetchers (default: 4)
• Checkpoint saved every N heights
• Progress reporting for observability (Observability: metrics, logging, and tracing #7)

Subscription manager (pkg/sync/subscriptions.go)

• Subscribes to new headers from upstream
• For each header, fetches blobs for tracked namespaces
• Detects gaps and triggers backfill mode
• Manages downstream subscriptions (see Configuration system: YAML loading and validation #13 for improvements)

Interface dependency

Consumes DataFetcher interface (pkg/fetch/fetcher.go):

type DataFetcher interface {
    GetHeader(ctx context.Context, height uint64) (*header.ExtendedHeader, error)
    GetBlobs(ctx context.Context, height uint64, ns Namespace) ([]*blob.Blob, error)
    GetNetworkHead(ctx context.Context) (uint64, error)
    SubscribeHeaders(ctx context.Context) (<-chan *header.ExtendedHeader, error)
    Close() error
}

References

• Design doc: test_plan.md — Sync Engine section