🐛 Fix zero-span issue for nearly collinear Voronoi inputs

[wwang721]

Summary

• When input points are nearly collinear, the Voronoi diagram may produce only one vertex, yielding a zero span in x or y
• The old code computed spans for vertices and points separately, which fails in this edge case
• Fix: stack vertices and points together by computing a unified bounding box (xmin, xmax, ymin, ymax) before deriving span_x, span_y, and span

Test plan

• Run existing test suite to confirm no regressions
• Manually test with three nearly collinear input points to confirm the zero-span case no longer errors or produces degenerate output

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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