feat: add verified snapshot manifests

[matthias-wright]

Changes:

• Add producer-side verification for Summit-backed snapshots:
  • Validate checkpoint digest against checkpoint data and finalized header
  • Decode last_block
  • Compare execution block number, block hash, and state root with Reth
• Generate epoch_N/manifest.json containing:
  • Summit checkpoint digest
  • Execution identity
  • Archive SHA-256 and size
  • Creation timestamp
• Expose manifests through:
  • GET /snapshots/{epoch}/manifest
• Pin compatible Summit SSZ and Commonware dependencies.