construct: accept rings pinched at a vertex on surface and 2D paths; refuse only where a solid would be non-manifold

[GeneralPawz]

Must land before #253 is released. #253 (on main, unreleased) replaced earcut with a certified ear clipper, which refuses rings that touch at exactly one vertex: a hole touching the outer ring or another hole, or an outer ring pinched at a vertex. That is right for solids, where extruding such a profile gives a non-manifold edge. But a consumer (axioval) legitimately produces such rings on surface and 2D paths:

• plan footprints that are unions of projected shadows, where two shadows touch at one corner (diagonal rectangles, an L-shaped room's boundary meeting a column corner);
• space-boundary surfaces.

They are listing the exact call sites.

Proposal

• Make pinch handling explicit per caller. Triangulating a 2D region or a planar surface patch accepts rings pinched at a vertex: split the pinch into separate ring pieces at the shared vertex, or let the clipper treat the pinch vertex as two coincident vertices. The output stays a valid triangulation whose edge parity holds when the pinch vertex is counted twice.
• Solid construction (extrusion caps and anything that must be a closed 2-manifold) keeps refusing by name. Alternatively, it emits two solids touching at an edge, but only where the caller asks for that.
• Keep the construct: profile triangulation leaves an open mesh for two holes side by side #253 certificate, adapted to pinch vertices.

Done when

• Pinched rings triangulate on the surface and 2D paths: diagonal rectangles touching at a corner, a hole touching the outer ring at a vertex, and two holes touching at a vertex, with exact area and a valid edge-parity certificate.
• Extrusion of the same profiles is refused by name.
• The consumer's listed call sites are covered by tests.

[GeneralPawz]

Released in axiolid-construct 0.3.15, axiolid-evaluate 0.3.6, axiolid-reference 0.3.7, axiolid-mesh-compile 0.3.14 (9877cdc).