- Add isDelete flag to VersionedRecord to fix COMMIT treating empty values
as DELETE. Previously version.xmax != TxnId(0) was never true for active
transactions, so deletes inside transactions never removed rows.
- Fix MIN/MAX aggregates skipping NULL: was checking for "\\N" instead
of correct "\N" sentinel, so NULL values were never skipped.
- Add duplicate alias deduplication in lowerSelect() and getSelectColumns().
Identical aliases now get _1, _2 suffixes (e.g. count(*), count(*)_1).
- Add nil guard for expr.subqueryPlan in irekSubquery evalExpr handler.
- Initialize txnManager in newExecutionContext to prevent segfault on BEGIN.
- Add regression tests: DELETE in transaction, PK-only row survival,
MIN/MAX NULL skipping.
