Remote UNION staging budget

Native remote UNION and UNION ALL share an additive allowance across all members: 100,000 input rows and 64 MiB of fixed-width row data. Each member is checked before its rows are copied into the UNION accumulator. A rejected member cursor is closed. Deduplication does not refund the allowance; even UNION of identical rows can be rejected. Local execution is unchanged.

The bound closes the repeated-member staging bypass of per-source SELECT preflight. It is conservative and does not cap total allocator overhead, individual memo sizes, expensive expressions, nested stages or statement series. These still need their own controls. File and protocol formats do not change. Use local execution or partition large trusted batch work.