refactor(topology): Rename Tet4 to Tetrahedron, remove hardcoded node count

- Changed struct name from Tet4 to Tetrahedron
- Removed nnodes() method (node count now determined by basis)
- Updated all function signatures to use Tetrahedron
- Added Tet4 as backwards compatibility alias
- Added note explaining basis determines node count (P1=4, P2=10 nodes)

src/topology/tet4.jl

@@ -2,9 +2,9 @@
 # License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE
 
 """
-    Tet4 <: AbstractTopology
+    Tetrahedron <: AbstractTopology
 
-4-node linear tetrahedral element.
+Tetrahedral element topology (3D simplex).
 
 Reference element in 3D parametric space with vertices at (0,0,0), (1,0,0), (0,1,0), (0,0,1).
 
@@ -25,22 +25,28 @@ N1-----|-----N3
        N2
 ```
 
+**Note:** Node count determined by basis function degree:
+- Lagrange{Tetrahedron, 1}: 4 nodes (P1)
+- Lagrange{Tetrahedron, 2}: 10 nodes (P2)
+
 **Zero allocation:** All functions return compile-time sized tuples.
 """
-struct Tet4 <: AbstractTopology end
+struct Tetrahedron <: AbstractTopology end
 
-nnodes(::Tet4) = 4
-dim(::Tet4) = 3
+dim(::Tetrahedron) = 3
+
+# Backwards compatibility alias
+const Tet4 = Tetrahedron
 
 """
-    reference_coordinates(::Tet4) -> NTuple{4, NTuple{3, Float64}}
+    reference_coordinates(::Tetrahedron) -> NTuple{4, NTuple{3, Float64}}
 
-Reference coordinates for 4-node tetrahedron:
+Reference coordinates for tetrahedron vertices:
 (0,0,0), (1,0,0), (0,1,0), (0,0,1).
 
 **Zero allocation:** Returns tuple of tuples (stack allocated).
 """
-reference_coordinates(::Tet4) = (
+reference_coordinates(::Tetrahedron) = (
     (0.0, 0.0, 0.0), # N1
     (1.0, 0.0, 0.0), # N2
     (0.0, 1.0, 0.0), # N3
@@ -48,25 +54,25 @@ reference_coordinates(::Tet4) = (
 )
 
 """
-    edges(::Tet4) -> NTuple{6, Tuple{Int, Int}}
+    edges(::Tetrahedron) -> NTuple{6, Tuple{Int, Int}}
 
 Edge connectivity for tetrahedron (6 edges).
 
 **Zero allocation:** Returns tuple of tuples (stack allocated).
 """
-edges(::Tet4) = (
+edges(::Tetrahedron) = (
     (1, 2), (2, 3), (3, 1),  # Base triangle
     (1, 4), (2, 4), (3, 4),  # Edges to apex
 )
 
 """
-    faces(::Tet4) -> NTuple{4, NTuple{3, Int}}
+    faces(::Tetrahedron) -> NTuple{4, NTuple{3, Int}}
 
 Face connectivity for tetrahedron (4 triangular faces).
 
 **Zero allocation:** Returns tuple of tuples (stack allocated).
 """
-faces(::Tet4) = (
+faces(::Tetrahedron) = (
     (1, 3, 2), # Base
     (1, 2, 4), # Front
     (2, 3, 4), # Right