nx_polygon.nx

nx_polygon.nx -- 2D polygon primitive with foundational ops + constant-distance Minkowski offset. Foundational substrate for the slicer pipeline (per NISHI_3D_PRINT_ROADMAP §2-3): every slice contour, every perimeter loop, every infill region is an NxPolygon manipulated through this primitive. Distinct from: nx_poly.nx -- polynomial algebra (p[0] + p[1]*x + ...) nx_geom.nx -- Hough transform (vision-pipeline geometry) nx_polyline -- (does not exist; if we add open polylines they go in a separate primitive) Coordinate convention: Q14 fixed-point xy. 1 Q14 unit = 1/16384. Matches nx_mesh + nx_stl pipeline so slice contours derived from triangle-plane intersections compose without unit conversion. Winding convention: CCW = solid (positive signed area), CW = hole (negative signed area). Same convention as nx_mesh (face front), most CAD systems, and the SVG / PDF / PostScript "fill rule". Per cardinal feedback-bits-up-exceed-never-match: composes the substrate's nx_isqrt (Q14 sqrt for unit-normal computation) + nx_i128 (overflow protection in line-line intersection). No CGAL, no libigl, no Boost.Geometry. Numerical robustness: - Foundational ops (area, point-in-poly, segment-intersect) use pure i64 arithmetic with cross-multiplication to avoid divides. - Coordinate magnitudes ≤ 325 mm * 16384 ≈ 2^23 (Qidi X-Max 3 build volume). Products of two coord-deltas ≤ 2^46. Sums of cross-products ≤ ~2^47. All fit in i64. - Offset uses nx_i128 for the t_num * edge_delta product in line-line intersection (would be 2^70 in i64, overflows). Per cardinal feedback-real-playtest-loop-not-just-logs: smoke passes are necessary but not sufficient. Offset on degenerate polygons (180-degree vertices, near-180 acute corners, near- collinear edges) needs print-evidence validation before claiming S-class.