feat(topology): Add Triangle 2D topology with Tri3/Tri6/Tri7 aliases

New file implementing 2D triangular topology:
- Triangle struct with dim=2, 3 corner nodes
- reference_coordinates() at (0,0), (1,0), (0,1)
- edges() returns 3 edges, faces() returns element itself
- Backward compatibility aliases: Tri3, Tri6, Tri7 (same topology, different basis)
- Zero-allocation tuple-based design
- Separation: topology is geometric shape, basis determines node count

src/topology/triangles.jl

@@ -0,0 +1,154 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+
+"""
+    Triangle <: AbstractTopology
+
+Triangular element topology in 2D (reference element geometry).
+
+**Important:** This type defines ONLY the geometric shape. Node count is determined
+by the interpolation scheme (basis functions):
+- `Lagrange{Triangle, 1}` → 3 nodes (P1, linear)
+- `Lagrange{Triangle, 2}` → 6 nodes (P2, quadratic)
+- `Lagrange{Triangle, 3}` → 10 nodes (P3, cubic)
+
+# Reference Element
+```
+     η
+     ^
+     |
+  (0,1) N3
+     |  \\
+     |    \\
+     |      \\
+     +---------> ξ
+  (0,0)    (1,0)
+   N1        N2
+```
+
+# Standard Corner Node Positions
+1. (0, 0) - Origin
+2. (1, 0) - Along ξ-axis
+3. (0, 1) - Along η-axis
+
+# Topology Properties
+- Dimension: 2
+- Corner nodes: 3
+- Edges: 3
+- Faces: 1 (the element itself in 2D)
+
+# Typical Usage
+```julia
+julia> topology = Triangle()
+julia> dim(topology)
+2
+julia> reference_coordinates(topology)  # Corner nodes only
+((0.0, 0.0), (1.0, 0.0), (0.0, 1.0))
+
+julia> basis = Lagrange{Triangle, 1}()
+julia> nnodes(basis)  # Node count from BASIS
+3
+
+julia> basis = Lagrange{Triangle, 2}()
+julia> nnodes(basis)  # Quadratic has 6 nodes (corners + edge midpoints)
+6
+```
+
+**Zero allocation:** All functions return compile-time sized tuples.
+
+See also: [`AbstractTopology`](@ref), [`Quadrilateral`](@ref), [`Lagrange`](@ref)
+"""
+struct Triangle <: AbstractTopology end
+
+dim(::Triangle) = 2
+
+"""
+    reference_coordinates(::Triangle)
+
+Get corner node positions for Triangle (3 vertices in parametric space).
+"""
+function reference_coordinates(::Triangle)
+    return (
+        (0.0, 0.0),  # Node 1: Origin
+        (1.0, 0.0),  # Node 2: Along ξ-axis
+        (0.0, 1.0),  # Node 3: Along η-axis
+    )
+end
+
+"""
+    edges(::Triangle)
+
+Edge connectivity for triangle (corner nodes).
+"""
+function edges(::Triangle)
+    return (
+        (1, 2),  # Edge 1: Bottom
+        (2, 3),  # Edge 2: Right (hypotenuse)
+        (3, 1),  # Edge 3: Left
+    )
+end
+
+"""
+    faces(::Triangle)
+
+For 2D elements, the face is the element itself.
+"""
+faces(::Triangle) = ((1, 2, 3),)
+
+# ============================================================================
+# Deprecated aliases (for backwards compatibility)
+# ============================================================================
+
+"""
+    Tri3
+
+**DEPRECATED:** Backward compatibility alias. Use `Triangle` with `Lagrange{Triangle, 1}`.
+
+The old `Tri3` conflated topology (triangle) with node count (3).
+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(Tri3, (1, 2, 3))
+# Internally converted to:
+element = Element(Triangle, (1, 2, 3))  # Infers Lagrange{Triangle, 1} from 3 nodes
+```
+
+New code should use explicit topology + basis:
+```julia
+element = Element(Lagrange{Triangle, 1}, (1, 2, 3))
+```
+"""
+const Tri3 = Triangle
+
+"""
+    Tri6
+
+**DEPRECATED:** Backward compatibility alias. Use `Triangle` with `Lagrange{Triangle, 2}`.
+
+This alias allows old code to work:
+```julia
+# Old style (still works)
+element = Element(Tri6, (1, 2, 3, 4, 5, 6))
+# Internally converted to:
+element = Element(Triangle, (1, 2, 3, 4, 5, 6))  # Infers Lagrange{Triangle, 2} from 6 nodes
+```
+
+New code should use explicit topology + basis:
+```julia
+element = Element(Lagrange{Triangle, 2}, (1, 2, 3, 4, 5, 6))
+```
+"""
+const Tri6 = Triangle  # Same topology! Node count from basis.
+
+"""
+    Tri7
+
+**DEPRECATED:** Backward compatibility alias. Use `Triangle` with `Lagrange{Triangle, 2}` + bubble.
+
+7-node triangle with interior bubble node.
+"""
+const Tri7 = Triangle  # Same topology! Node count from basis.