The single memory ceiling every buffering subsystem derives its working budget
from: the external sort's spill threshold, the self-overlay tile cap, spatial
run-file flushes, partition routing, and the spilling hash join. Resolution
order is an explicit limit, then getOption("vectra.memory"), then a default
of half the detected system RAM. The auto-detected default is floored at 1 GB;
an explicit limit or option is honored as given (down to 1 KB), so a smaller
budget can be requested deliberately.