LastSaveLargestMemoryBuffer property

Size in bytes of the largest in-memory buffer the most recent save held.

Reports the high-water mark of save staging, which is the measurement to size SaveMemoryLimit against instead of guessing from document size

Together with LastSaveSpillCount it distinguishes "close to the limit" from "well over it", which decides whether raising the limit would actually avoid the spill

The value is reset at the start of each save operation

SaveMemoryLimit, LastSaveSpillCount, SaveTemporaryDirectory