Excavation Unit Current Depth

The currentDepth of an ExcavationUnit is the running record of how far down you have dug, updated as work proceeds. It starts at zero when the unit is first opened and increases as you remove soil and complete each StratigraphicLayer, giving everyone a quick read on the state of the unit without having to flip through level forms. Read against the datumElevation, the current depth converts to a real elevation, and it pairs naturally with the unit's status — a unit moving from IN_PROGRESS toward CLOSED will typically have reached its target depth. Because it defaults to zero, a freshly created unit always shows a sensible starting value.