Function computeLabelPlacement

Approximate the pole-of-inaccessibility for a triangle-soup polygon, returned in projection-basis (u, v) space.

Strategy:

1. Project every position from world space onto the ProjectionBasis's (right, up) plane, building a u/v point list parallel to the original triangle indices.
2. Walk the triangle list, summing the triangulated area (shoelace per triangle), and collect every triangle edge into a multiset. Edges shared between two triangles are interior; edges that appear exactly once are boundary. The boundary edge set is the polygon outline regardless of triangulation choice.
3. Grid-sample the polygon's (u, v) AABB. At each sample point, run the standard winding-number point-in-polygon test against the boundary edges; if inside, compute the minimum distance to any boundary edge. The sample with the largest min-distance is the placement anchor; that distance is the inscribed radius.

This is a coarse-grid approximation of the recursive subdivision used in Mapbox's polylabel — accurate to one cell at the chosen grid resolution, which is sub-decimetre at the default 32×32 grid on a typical 5–15 m room. Fine for label placement; not fine enough to drive a CAM tool.

Returns null when the polygon has no boundary edges (every edge was shared by two triangles — degenerate input) or no grid sample landed inside.