Add voxel-edge contour mode

[carluri]

Summary

Adds an opt-in voxel-edge contour strategy when converting binary masks to
RTSTRUCT contours.

rtstruct.add_roi(mask=mask, name="Kidney", contour_mode="voxel_edge")

The existing voxel_center behavior remains the default for backward
compatibility.

Why

The existing OpenCV contour extraction traces through the centres of boundary
voxels. When another application rasterizes those polygons using voxel-centre
inclusion, the outermost voxel ring can be lost and the resulting ROI appears
inset by half a voxel.

The new mode traces the boundary of the union of foreground voxel squares at
half-integer pixel coordinates. This places contours on the outer voxel edges
and allows consumers using voxel-centre inclusion to reproduce the complete
mask footprint.

Changes

• Add contour_mode to RTStruct.add_roi and ROIData.
• Preserve contour_mode="voxel_center" as the default.
• Add native voxel_edge polygon tracing with collinear-point removal.
• Validate unsupported contour modes.
• Document the new API in the README.
• Test single voxels, rectangles, holes, diagonal contact, invalid modes, and
  patient-space placement.

Validation

• pytest -q
• 45 passed
• 72 existing pydicom deprecation warnings

[qurit-frizi]

Approved.