Stabilize voxel boundary binning across platforms in VoxelGrid

[Copilot]

The macOS 3.11 Actions job was failing due to platform-dependent voxel index assignment at segment boundaries. Boundary points were mapped differently across environments because voxel segments inherited lower-precision numeric behavior.

• Root cause

  • Voxel segment edges were generated from point-derived bounds without enforcing a stable precision, causing boundary classification drift in np.searchsorted paths on some platforms.

• Code change

  • In VoxelGrid.compute, segment generation now uses explicit float64 bounds and output:
    • float(xyzmin[i]), float(xyzmax[i])
    • dtype=np.float64 in np.linspace
  • This keeps voxel boundary construction deterministic while preserving existing indexing logic.

• Coverage update

  • Added a focused unit test for boundary-point query assignment to lock in expected behavior for points exactly on representative segment edges.

s, step = np.linspace(
    float(xyzmin[i]),
    float(xyzmax[i]),
    num=(self.x_y_z[i] + 1),
    retstep=True,
    dtype=np.float64,
)