feat(topology): Add Pyramid 3D topology with Pyr5 alias

New file implementing 3D pyramidal topology:
- Pyramid struct with dim=3, 5 corner nodes (square base + apex)
- reference_coordinates() with base at z=0 and apex at (0,0,1)
- edges() returns 8 edges (4 base + 4 to apex)
- faces() returns 5 faces (1 quadrilateral base + 4 triangular sides)
- Backward compatibility alias: Pyr5 (linear)
- Zero-allocation tuple-based design

src/topology/pyramids.jl

@@ -0,0 +1,112 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE
+
+"""
+    Pyramid <: AbstractTopology
+
+Pyramidal element topology (3D).
+
+**Important:** This type defines ONLY the geometric shape. Node count is determined
+by the interpolation scheme (basis functions):
+- `Lagrange{Pyramid, 1}` → 5 nodes (linear)
+- `Lagrange{Pyramid, 2}` → 13 nodes (quadratic, with edge midpoints)
+
+# Reference Element
+```
+        N5 (apex)
+        /|\\
+       / | \\
+      /  |  \\
+     /   |   \\
+    N4---+---N3
+    |    |   |
+    |    N1  |
+    |   /    |
+    |  /     |
+    | /      |
+    N1------N2
+```
+
+# Standard Corner Node Positions
+1-4: Square base in z=0 plane
+5: Apex
+
+# Topology Properties
+- Dimension: 3
+- Corner nodes: 5
+- Edges: 8
+- Faces: 5 (1 quad + 4 triangles)
+
+**Zero allocation:** All functions return compile-time sized tuples.
+
+See also: [`AbstractTopology`](@ref), [`Wedge`](@ref)
+"""
+struct Pyramid <: AbstractTopology end
+
+dim(::Pyramid) = 3
+
+"""
+    reference_coordinates(::Pyramid)
+
+Get corner node positions for Pyramid (5 vertices).
+"""
+function reference_coordinates(::Pyramid)
+    return (
+        (-1.0, -1.0, 0.0),  # Node 1: Base corner
+        (1.0, -1.0, 0.0),  # Node 2: Base corner
+        (1.0, 1.0, 0.0),  # Node 3: Base corner
+        (-1.0, 1.0, 0.0),  # Node 4: Base corner
+        (0.0, 0.0, 1.0),  # Node 5: Apex
+    )
+end
+
+"""
+    edges(::Pyramid)
+
+Edge connectivity for pyramid (corner nodes).
+"""
+function edges(::Pyramid)
+    return (
+        (1, 2), (2, 3), (3, 4), (4, 1),  # Base edges
+        (1, 5), (2, 5), (3, 5), (4, 5),  # Edges to apex
+    )
+end
+
+"""
+    faces(::Pyramid)
+
+Face connectivity for pyramid (1 quad base + 4 triangular faces).
+"""
+function faces(::Pyramid)
+    return (
+        (1, 4, 3, 2),  # Face 1: Quadrilateral base
+        (1, 2, 5),     # Face 2: Triangle
+        (2, 3, 5),     # Face 3: Triangle
+        (3, 4, 5),     # Face 4: Triangle
+        (4, 1, 5),     # Face 5: Triangle
+    )
+end
+
+# ============================================================================
+# Deprecated aliases (for backwards compatibility)
+# ============================================================================
+
+"""
+    Pyr5
+
+**DEPRECATED:** Backward compatibility alias. Use `Pyramid` with `Lagrange{Pyramid, 1}`.
+
+This alias allows old code to work:
+```julia
+# Old style (still works)
+element = Element(Pyr5, (1, 2, 3, 4, 5))
+# Internally converted to:
+element = Element(Pyramid, (1, 2, 3, 4, 5))  # Infers Lagrange{Pyramid, 1}
+```
+
+New code should use explicit topology + basis:
+```julia
+element = Element(Lagrange{Pyramid, 1}, (1, 2, 3, 4, 5))
+```
+"""
+const Pyr5 = Pyramid