feat(test): First test rewritten for immutable elements (test_elasticity_1d)

Rewrote test_elasticity_1d.jl to follow immutable element pattern.
This is the first fully working test with the new architecture!

Changes:
1. test/test_elasticity_1d.jl:
   - Convert Dict node data to element-local tuple format
   - Wrap data in DVTI field objects (Discrete, Variable, Time-Invariant)
   - Create element with fields at construction: Element(Seg2, conn; fields=(...))
   - Fix Jacobian shape expectation (3×1 not 1×3 for 1D in 3D)

2. src/JuliaFEM.jl:
   - Add minimal jacobian() function for AbstractBasis (non-parametric)
   - Handles embedding (1D element in 3D space) correctly
   - Returns Matrix instead of Tensor for flexibility

3. src/elements/elements.jl:
   - Fix Jacobian computation to handle both Tuple and IntegrationPoint
   - Fix detJ calculation logic for embedded elements (check m not size(JT,2))
   - Correctly handle 1D elements: detJ = ||∂X/∂ξ||

Result: test_elasticity_1d.jl passes! ✓

This validates the immutable architecture:
- Element created with fields at construction
- No mutation needed during test
- Field system integration working (DVTI fields)
- Jacobian computation working for embedded elements

test/test_elasticity_1d.jl

@@ -5,15 +5,27 @@ using JuliaFEM, Test
 
 # 1d strain
 
-X = Dict(1 => [0.0, 0.0, 0.0], 2 => [1.0, 1.0, 1.0])
-u = Dict(1 => [0.0, 0.0, 0.0], 2 => [1.0, 1.0, 1.0])
-element = Element(Seg2, (1, 2))
-update!(element, "geometry", X)
-update!(element, "displacement", u)
+# Global node data (Dict format for backward compatibility in tests)
+X_global = Dict(1 => [0.0, 0.0, 0.0], 2 => [1.0, 1.0, 1.0])
+u_global = Dict(1 => [0.0, 0.0, 0.0], 2 => [1.0, 1.0, 1.0])
+
+# Convert to element-local format (extract data for element nodes)
+connectivity = (1, 2)
+X = tuple([X_global[i] for i in connectivity]...)
+u = tuple([u_global[i] for i in connectivity]...)
+
+# Wrap in field objects (DVTI = Discrete, Variable, Time-Invariant)
+X_field = JuliaFEM.DVTI(X)
+u_field = JuliaFEM.DVTI(u)
+
+# Create element with fields at construction (immutable pattern)
+element = Element(Seg2, connectivity; fields=(geometry=X_field, displacement=u_field))
+
 xi, time = (0.0,), 0.0
 detJ = element(xi, time, Val{:detJ})
 J = element(xi, time, Val{:Jacobian})
 # gradu = element("displacement", xi, time, Val{:Grad})
-@debug("1d seg2 info", xi ,time, detJ, J)
+@debug("1d seg2 info", xi, time, detJ, J)
 @test isapprox(detJ, sqrt(3)/2)
-@test isapprox(J, [0.5 0.5 0.5])
+# Jacobian is 3×1 (physical_dim × parametric_dim) for 1D element in 3D
+@test isapprox(J, [0.5; 0.5; 0.5])  # column vector