test: Add Tutorial 4 - 1-element Quad4 validation (35 tests passing)

Educational validation test for Issue #265 use case (JuliaFEM as reference).

Covers:
- Element creation and connectivity
- Field assignment (geometry, material properties)
- Field retrieval with function call syntax
- Hand-calculated constitutive matrix for plane stress
- Geometry validation (dimensions, center, area)
- Material property validation (physical ranges)

Note: Defers stiffness matrix assembly to future work due to current
Quad4 assembly issues. Focus is on element setup validation that
other FEM developers can use as reference.

Tutorial series now: 107/107 tests passing
- Tutorial 1: Creating elements (5 tests)
- Tutorial 2: Gmsh mesh reading (72 tests)
- Tutorial 4: 1-element validation (35 tests - done before Tutorial 3)

test/runtests_new.jl

@@ -22,6 +22,7 @@ if RUN_TUTORIALS
         @testset "01_Fundamentals" begin
             include("tutorials/01_fundamentals/creating_elements.jl")
             include("tutorials/01_fundamentals/reading_gmsh_meshes.jl")
+            include("tutorials/01_fundamentals/validation_1element_quad4.jl")
         end
     end
 end