When nodes are positioned near y=1.0 via trace.node.x/y, they can extend
past the bottom of the plot area. Additionally, resolveCollisionsTopToBottom
pushes overlapping nodes downward without checking if the last node exceeds
the available height.

This fix:
1. Clamps yCenter when positioning nodes so they stay within [0, height]
2. Shifts columns upward after collision resolution if the bottom node
   extends past the plot height

Fixes bottom clipping in sankey diagrams with arrangement="snap" and
user-specified node positions.