feat: external block provider for genesis sync

[tac0turtle]

Summary

Nodes running in pruning mode (default) discard old blocks to bound disk growth. To support syncing new nodes from genesis without requiring every node to be a full archive node, we need an external block serving service.

Motivation

• Block archive storage now prunes by default (retention window of ~100k blocks)
• Chain-index (SQLite) serves RPC/explorer queries independently of the block archive
• Only seed/archive nodes need full block history
• New nodes joining the network need a way to sync from genesis without requiring every peer to store all blocks

Design

An external block provider service that:

1. Stores all blocks from genesis (potentially using commonware's immutable::Archive with freezer for minimal memory + compression)
2. Serves block data over a well-defined API (HTTP/gRPC) to syncing nodes
3. Can be operated independently of the P2P network -- e.g., a centralized service, cloud storage, or a dedicated archive node

Sync flow

New Node                    Block Provider
  |                              |
  |-- request blocks [0, N) --->|
  |<--- stream ArchivedBlock ---|
  |                              |
  |-- apply blocks to STF ---   |
  |-- build chain-index ---     |
  |                              |
  |-- join P2P, continue from  |
  |   latest block              |

Considerations

• The provider does not need to be trusted for correctness -- the node validates state roots against the chain's state transition function
• Provider could be backed by object storage (S3/R2) serving pre-compressed block segments
• Protocol should support range requests and resumable sync
• Consider using commonware's immutable::Archive (freezer) as the backing store for the provider -- it's designed for exactly this workload (write-once, read-many, minimal memory, compressed)

Acceptance Criteria

• Define block provider API (gRPC service definition or HTTP endpoints)
• Implement block provider server backed by full block archive
• Implement client-side sync that fetches blocks from provider and applies them
• Support resumable sync (track last synced block)
• Node config option to specify block provider URL for initial sync