storage: define pruning/snapshots/compaction strategy for mainnet

[TheNewAutonomy]

Goal

Define and implement a production strategy for state storage growth: pruning, snapshots, compaction, and safe migrations.

Scope

• Decide retention policy (full history vs pruned) and what is required to verify state
• Snapshot format + restore procedure
• RocksDB compaction/CF tuning and disk growth monitoring
• Data migrations/versioning for on-disk formats

Acceptance criteria

• Node can run for long periods without unbounded disk growth (with a defined policy)
• Snapshot/restore can bring up a new node to current head
• Migration path is tested and documented