sync: snapshot advertisement format + RPC discoverability #226
Description
Goal
Make snapshots discoverable and verifiable so a brand-new node can choose a trusted snapshot source and validate it before restore.
Scope
- Define snapshot metadata schema (archive URL, sha256, byte size, created_at, chain/network id, state root / cycle, optional TTL)
- RPC endpoints to fetch current snapshot advertisement(s)
- Storage format in node DB for published snapshot metadata (if not already consistent)
- Document operator flow (publisher + consumer)
Acceptance criteria
- A consumer node can query an RPC endpoint and obtain a complete, self-contained snapshot advertisement
- Snapshot metadata includes integrity fields (hash + size) and chain identity binding
- Multiple snapshot publishers can coexist; client can select based on recency/health
Refs: existing CLI commands snapshot-publish and sync-from-snapshot should be aligned to the same metadata shape.