refactor(topology): Rename Tri3 to Triangle, remove hardcoded node count

- Changed struct name from Tri3 to Triangle
- Removed nnodes() method (node count now determined by basis)
- Updated documentation to explain topology vs basis separation
- Added examples showing Lagrange{Triangle, P} for different degrees
- Added Tri3 as deprecated alias for backwards compatibility
- Clarified that topology defines geometry only, basis determines nodes

src/topology/tri3.jl

@@ -2,9 +2,16 @@
 # License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
 
 """
-    Tri3 <: AbstractTopology
+    Triangle <: AbstractTopology
 
-Three-node triangular element in 2D.
+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)
+- `Nedelec{Triangle, 1}` → 3 nodes, but DOFs on edges
 
 # Reference Element
 ```
@@ -19,49 +26,83 @@ Three-node triangular element in 2D.
   (0,0)    (1,0)
 ```
 
-# Node Ordering
-Nodes are numbered counter-clockwise starting from origin:
+# Standard Node Positions
+For `Lagrange{Triangle, 1}` (P1, 3 nodes):
 1. (0, 0) - Origin
 2. (1, 0) - Along ξ-axis
 3. (0, 1) - Along η-axis
 
-# Properties
-- Nodes: 3
+For higher-order bases, additional nodes are added on edges and interior.
+
+# Topology Properties
 - Dimension: 2
 - Edges: 3
-- Faces: 1 (the element itself)
+- Faces: 1 (the element itself in 2D)
 
 # Typical Usage
 ```julia
-julia> topology = Tri3()
-julia> nnodes(topology)
-3
+julia> topology = Triangle()
 julia> dim(topology)
 2
+julia> basis = Lagrange{Triangle, 1}()
+julia> nnodes(basis)  # Node count comes from BASIS, not topology
+3
 ```
 
-See also: [`AbstractTopology`](@ref), [`Tri6`](@ref), [`Quad4`](@ref)
+See also: [`AbstractTopology`](@ref), [`Quadrilateral`](@ref), [`Lagrange`](@ref)
 """
-struct Tri3 <: AbstractTopology end
+struct Triangle <: AbstractTopology end
 
-nnodes(::Tri3) = 3
-dim(::Tri3) = 2
+dim(::Triangle) = 2
 
-function reference_coordinates(::Tri3)
+"""
+    reference_coordinates(::Triangle)
+
+Get standard reference node positions for Triangle (3 vertices in parametric space).
+These are the P1 (linear) node positions by default.
+"""
+function reference_coordinates(::Triangle)
     return (
-        (0.0, 0.0),  # Node 1
-        (1.0, 0.0),  # Node 2
-        (0.0, 1.0),  # Node 3
+        (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
 
-function edges(::Tri3)
+function edges(::Triangle)
     return (
         (1, 2),  # Edge 1: Bottom
-        (2, 3),  # Edge 2: Right
+        (2, 3),  # Edge 2: Right (hypotenuse)
         (3, 1),  # Edge 3: Left
     )
 end
 
 # For 2D elements, faces are the element itself
-faces(::Tri3) = ((1, 2, 3),)
+faces(::Triangle) = ((1, 2, 3),)
+
+# ============================================================================
+# Deprecated aliases (for backwards compatibility)
+# ============================================================================
+
+"""
+    Tri3
+
+**DEPRECATED:** Use `Triangle` + `Lagrange{Triangle, 1}` instead.
+
+The old `Tri3` type conflated topology (triangle) with node count (3).
+In the new architecture:
+- Topology defines geometric shape only
+- Basis functions determine node count
+
+Migration:
+```julia
+# Old (deprecated)
+element = Element(Tri3, (1,2,3))
+
+# New (correct)
+element = Element(Triangle(), Lagrange{Triangle, 1}(), Gauss{2}(), (1,2,3))
+```
+"""
+const Tri3 = Triangle
+
+# Note: Tri3 is deprecated. Use Triangle with parametric Lagrange basis instead.