perf(gateway): share one group event snapshot per transaction across recipients #607

This was generated by AI during triage.

PR #605 made the group event relay send one transaction's rows in insert order. For that, wire_group_events (gateway/src/stdb/world_view.rs) reads the cached game_group_event table once per transaction per recipient with a session. A transaction with many recipients, such as a 40-member Raid roster list or a loot roll, repeats the same table copy on the shared coordinator pump.

The event GC holds rows for about one second, so the table stays small and the cost has a ceiling. The work still grows with recipients × cached rows for every burst.

Done when: each source database collects one transaction's rows once, and each recipient takes its slice from that snapshot. The ordering guarantee and the per-database cursor from #605 stay the same. A test proves one table read per transaction for many recipients.

Note: the existing by_recipient index is on recipient_identity, not recipient_guid, so it does not serve this lookup directly.

Raised by CodeRabbit on #605.