Windows deletes a VSS snapshot once the old copies of rewritten blocks
exceed the volume's shadow-storage cap, which defaults to ~1% of the disk
(17.8 GB on a 1.8 TB drive). A long rehearsal with other disk activity can
blow that and cost the tester the rollback.
pre now raises the cap to 128 GB (a ceiling, not a reservation) right after
taking the snapshot, recording the exact original in shadowstorage.txt; post
puts it back, also on the snapshot-gone path. An already-larger cap is left
alone.
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