Status: IN
`MVCCDatabase.begin_transaction` captures `active_at_start` as a frozen set of all currently active transaction IDs at that moment; it is never modified after creation and serves as the transaction's immutable snapshot boundary
Source: entries/2026/05/29/topic-snapshot-vs-ssi-tradeoffs.md
tx.active_at_start = {
tid for tid, t in self._transactions.items() if t.is_active}