feat(vortex-geo): add convex hull scalar function

[HarukiMoriarty]

Summary

• add a strict native MultiPoint to Polygon convex-hull scalar function
• delegate planar hull computation directly to geo::ConvexHull
• preserve CRS and outer nullability while materializing native 2-D Polygon storage
• use shared unary dispatch for constants and strict null propagation
• cover empty, one-point, collinear, nullable, constant, and Q5-style collect-to-hull-to-area execution
• add a dedicated convex-hull microbenchmark

Semantics

The return dtype is always native Polygon, including empty and degenerate hulls. Consistent with the current geometry-kernel policy, coordinate finiteness is unchecked unless validation is explicitly enabled; this function does not add per-call NaN validation.