Function currentFloorBands

Compute the construction-projection band depths for the storey containing the cut, given the per-storey floor levels (ascending) and the model's axis extent — all along the cut axis, in the same render-frame metric space as position.

The current storey k is the highest storey whose floor is at or below the cut. Its vertical slab is [floor_k, ceil_k):

• the bottom storey extends DOWN to the model bottom (axisMin) so site / foundation / terrain below the ground floor still projects (solid);
• the top storey extends UP to the model top (axisMax) so the roof / eaves above the top storey still projects (dashed);
• an intermediate storey is clamped to the next storey's floor (its ceiling), so the floor above is culled.

The returned depths feed ProjectionBandDepths unchanged: below becomes the VISIBLE/solid band (toward the floor) and above the OVERHEAD/dashed band. Every depth is floored at minDepth so a cut sitting exactly on a storey boundary never yields a zero-width band that culls everything on the plane.