nx_slice_contour.nx

nx_slice_contour.nx -- assemble nx_slice_plane segment soup into closed NxPolygon contours via endpoint-adjacency walk. Algorithm (per layer): for each unvisited segment S: start a polygon with S.A as first vertex, S.B as second mark S visited current_endpoint = S.B loop: find another unvisited segment T whose endpoint matches current_endpoint if T found: add T's OTHER endpoint to polygon mark T visited current_endpoint = T's other endpoint else: contour is OPEN (dangling segment chain) -- emit as open polygon with verdict set; slicer caller decides handling if current_endpoint == S.A (start): contour CLOSED -- emit as closed polygon end for Complexity: O(n²) per layer. At slicer-typical 10K segments/layer this is ~100M ops -- well within seconds for a 100-layer slice. Per cardinal feedback-bits-up-exceed-never-match: O(n) hash-based version queued as P2.2.opt for after this primitive's semantics have been compile-validated. Correctness first, perf second -- per "bad prints = massive waste of time and resources" directive. Composes nx_slice_plane (segment soup input) + nx_polygon (closed contour output). No new substrate primitives needed. Manifold expectation: a well-formed closed STL produces a soup where every endpoint is shared by EXACTLY two segments. The walk terminates by returning to the start vertex. Non-manifold or open-mesh input produces dangling chains and gets flagged via NX_SLICE_CONTOUR_ERR_OPEN. license_tier: ORIGINAL