Prune VSS store by reachable sessions

[zjma]

Summary

• add VSS store pruning by live session address set instead of the unused epoch column
• derive live VSS sessions from current and previous on-chain network secrets when the network is stable
• prune stale dealer/share rows after stable epoch observation and add coverage for epoch-independent pruning

Tests

• cargo test -p vss-store
• cargo test -p network-node