feat(topology): Add Tetrahedron 3D topology with Tet4/Tet10 aliases

New file implementing 3D tetrahedral topology:
- Tetrahedron struct with dim=3, 4 corner nodes (3D simplex)
- reference_coordinates() at (0,0,0), (1,0,0), (0,1,0), (0,0,1)
- edges() returns 6 edges, faces() returns 4 triangular faces
- Backward compatibility aliases: Tet4 (linear), Tet10 (quadratic)
- Zero-allocation tuple-based design

src/topology/tetrahedra.jl

@@ -0,0 +1,156 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE
+
+"""
+    Tetrahedron <: AbstractTopology
+
+Tetrahedral element topology (3D simplex).
+
+**Important:** This type defines ONLY the geometric shape. Node count is determined
+by the interpolation scheme (basis functions):
+- `Lagrange{Tetrahedron, 1}` → 4 nodes (P1, linear)
+- `Lagrange{Tetrahedron, 2}` → 10 nodes (P2, quadratic)
+- `Lagrange{Tetrahedron, 3}` → 20 nodes (P3, cubic)
+
+# Reference Element
+```
+      N4
+      /|\\
+     / | \\
+    /  |  \\
+   /   |   \\
+  N1---+----N3
+   \\  /
+    \\/
+    N2
+```
+
+# Standard Corner Node Positions
+1. (0, 0, 0) - Origin
+2. (1, 0, 0) - Along ξ-axis
+3. (0, 1, 0) - Along η-axis
+4. (0, 0, 1) - Along ζ-axis
+
+# Topology Properties
+- Dimension: 3
+- Corner nodes: 4
+- Edges: 6
+- Faces: 4 (triangular)
+
+# Typical Usage
+```julia
+julia> topology = Tetrahedron()
+julia> dim(topology)
+3
+julia> reference_coordinates(topology)  # Corner nodes only
+((0.0, 0.0, 0.0), (1.0, 0.0, 0.0), (0.0, 1.0, 0.0), (0.0, 0.0, 1.0))
+
+julia> basis = Lagrange{Tetrahedron, 1}()
+julia> nnodes(basis)  # Linear: 4 nodes
+4
+
+julia> basis = Lagrange{Tetrahedron, 2}()
+julia> nnodes(basis)  # Quadratic: 10 nodes (corners + edge midpoints)
+10
+```
+
+**Zero allocation:** All functions return compile-time sized tuples.
+
+See also: [`AbstractTopology`](@ref), [`Hexahedron`](@ref), [`Lagrange`](@ref)
+"""
+struct Tetrahedron <: AbstractTopology end
+
+dim(::Tetrahedron) = 3
+
+"""
+    reference_coordinates(::Tetrahedron)
+
+Get corner node positions for Tetrahedron (4 vertices in parametric space).
+"""
+function reference_coordinates(::Tetrahedron)
+    return (
+        (0.0, 0.0, 0.0),  # Node 1: Origin
+        (1.0, 0.0, 0.0),  # Node 2: Along ξ-axis
+        (0.0, 1.0, 0.0),  # Node 3: Along η-axis
+        (0.0, 0.0, 1.0),  # Node 4: Along ζ-axis
+    )
+end
+
+"""
+    edges(::Tetrahedron)
+
+Edge connectivity for tetrahedron (corner nodes).
+"""
+function edges(::Tetrahedron)
+    return (
+        (1, 2),  # Edge 1
+        (2, 3),  # Edge 2
+        (3, 1),  # Edge 3
+        (1, 4),  # Edge 4
+        (2, 4),  # Edge 5
+        (3, 4),  # Edge 6
+    )
+end
+
+"""
+    faces(::Tetrahedron)
+
+Face connectivity for tetrahedron (triangular faces, corner nodes).
+"""
+function faces(::Tetrahedron)
+    return (
+        (1, 3, 2),  # Face 1: Base (looking from above)
+        (1, 2, 4),  # Face 2
+        (2, 3, 4),  # Face 3
+        (3, 1, 4),  # Face 4
+    )
+end
+
+# ============================================================================
+# Deprecated aliases (for backwards compatibility)
+# ============================================================================
+
+"""
+    Tet4
+
+**DEPRECATED:** Backward compatibility alias. Use `Tetrahedron` with `Lagrange{Tetrahedron, 1}`.
+
+The old `Tet4` conflated topology (tetrahedron) with node count (4).
+In the new architecture:
+- Topology defines geometric shape only
+- Basis functions determine node count
+
+This alias allows old code to work:
+```julia
+# Old style (still works)
+element = Element(Tet4, (1, 2, 3, 4))
+# Internally converted to:
+element = Element(Tetrahedron, (1, 2, 3, 4))  # Infers Lagrange{Tetrahedron, 1}
+```
+
+New code should use explicit topology + basis:
+```julia
+element = Element(Lagrange{Tetrahedron, 1}, (1, 2, 3, 4))
+```
+"""
+const Tet4 = Tetrahedron
+
+"""
+    Tet10
+
+**DEPRECATED:** Backward compatibility alias. Use `Tetrahedron` with `Lagrange{Tetrahedron, 2}`.
+
+This alias allows old code to work:
+```julia
+# Old style (still works)
+element = Element(Tet10, (1, 2, 3, 4, 5, 6, 7, 8, 9, 10))
+# Internally converted to:
+element = Element(Tetrahedron, (1, 2, 3, 4, 5, 6, 7, 8, 9, 10))  # Infers Lagrange{Tetrahedron, 2}
+```
+
+New code should use explicit topology + basis:
+```julia
+element = Element(Lagrange{Tetrahedron, 2}, (1, 2, 3, 4, 5, 6, 7, 8, 9, 10))
+```
+"""
+const Tet10 = Tetrahedron  # Same topology! Node count from basis.