Fix gabriel_graph

[yu-ta-sato]

Description

This pull request fixes the Gabriel graph edge filtering logic in city2graph.proximity.gabriel_graph.

Previously, the implementation tested whether points fell inside the disc with diameter uv by computing distances from each point to the midpoint of the candidate edge. With large projected-coordinate values, this midpoint/radius comparison could become numerically fragile and incorrectly drop valid Gabriel edges.

The fix replaces that predicate with an equivalent dot-product test:

• a third point lies strictly inside the open Gabriel disc for edge (u, v) iff
  dot(u - w, v - w) < 0
• candidate Delaunay edges are kept only when no point satisfies that condition

The docstring was also updated to describe the open-disc definition explicitly.

A regression test was added using large projected-CRS-like coordinates. The test verifies that:

• gabriel_graph matches a brute-force Gabriel graph definition
• the expected graph hierarchy holds: MST ⊆ RNG ⊆ Gabriel ⊆ Delaunay

Related Issues

N/A

Checklist

• I have read the Contributing Guide.
• I have updated the documentation, if necessary.
• I have added tests to cover my changes.
• All new and existing tests passed.
• Pre-commit checks passed locally.