Viewport association fixture

viewport_associations.dxf contains line, circle, arc, polyline, spline, block-instance, and intersection references through a 1:2 viewport. src/app/viewport_dimension_tests.rs tests refresh and intersection source edits.

Each dimension stores DIMLFAC=-2 and ACAD_DIMASSOC_CALC_DIMLFAC=-2 so its paper definition points measure in model units. The spline tangent from (0, 300) touches (90, 283.125) at parameter 0.75; tests verify its tangency and preserve the dimension if an edited reference cannot resolve.