Define Serialization format with flat hierarchy.

[codingminions]

Context

• The spec demands "smallest possible overhead" on the Serialize output.
• The blob is what gets stored per paused sandbox, potentially in object storage for weeks. Format choices here have downstream consequences for cost and compatibility.

Decision

Flat repeated entries, no header, no trailer:

[uint64 big-endian block#][4096 bytes raw data]

Entries sorted ascending by block number. No duplicates.