Improvements in arrangement processing

[Ylannl]

1 Fixes roof-ground bowties

after - before:

• Introduces ElevationProvider into ArrangementDissolver.
• Clips arrangement roof faces where the assigned roof plane intersects terrain.
• Marks below-terrain subfaces as ground by setting in_footprint = false and is_ground = true.
• Marks enclosed below-terrain regions as is_footprint_hole so floor holes are preserved.
• Keeps existing outside-footprint dissolution behavior, so exterior-connected ground merges out as before.
• Uses CGAL plane/triangle intersections for interpolated terrain cuts.
• Prevents ArrangementExtruder from extruding roof faces below terrain, avoiding bowtie/invalid geometry.

2 Prevent non manifold edges in extrusion

before - after:

Fix non-manifold extrusion edges caused by alternating roof heights at high-valence arrangement vertices.

• Detect problematic junctions in the snapper’s final CDT.
• Replace each junction with an adaptive-radius chord polygon.
• Merge the repair area into the lowest incident roof face.
• Transfer face metadata using final constrained CDT regions.
• Add regression tests verifying valid junction preservation and 2-manifold extrusion.

3 Prevent self intersecting polygons

This in some cases also led to holes in the adjacent wall faces. Fixed using the same approach as for 2, ie. extends the criteria to detect problematic junctions by counting how many times the same face is incident, and joins the repair area in that same face.

before - after: