Plot-ready geometry extraction

Geo(model) (or Geo(network)) flattens a solved structure into plain arrays for plotting — node positions, displaced positions, element connectivity, and per-element force ranges with their maxima. ElementDisplacements(element, model) samples the exact deflected curve along a member.

The examples reuse the truss (a solved Warren2D) and bldg (a solved Frame) from Structure generators:

geo = Geo(truss.model)          # geo.nodes, geo.disp, geo.indices, geo.P, …

ModelGeo  (plot-ready arrays from a solved frame model)
  21 nodes, 39 elements
  max |u|  = 0.028357988559118953  [length]         (largest nodal displacement)
  max |N|  = 187.49999999999994  [force]          (largest axial end force)
  max |M|  = 0.0  [force·length]   (largest bending end moment)
  fields: nodes, disp, indices, P, Vy, Vz, Tx, My, Mz, areas, lengths, …

ed = ElementDisplacements(bldg.model.elements[1], bldg.model; resolution = 20)
ed.basepositions .+ 100 .* ed.uglobal   # 3×20 displaced curve, scaled ×100

3×20 Matrix{Float64}:
 0.0  -0.000536406  -0.00106384  -0.00157332  …  -0.00095896   3.92842e-5
 0.0  -3.58372e-6   -6.83631e-6  -9.4266e-6       0.000256371  0.000309413
 0.0   0.210477      0.420954     0.631431        3.78859      3.99907