refactor(topology): Rename Quad4 to Quadrilateral, remove hardcoded node count

- Changed struct name from Quad4 to Quadrilateral
- Removed nnodes() method (node count now determined by basis)
- Updated documentation to explain topology vs basis separation
- Added examples showing Lagrange and Serendipity differences
- Added Quad4 as deprecated alias for backwards compatibility
- Clarified that topology defines geometry only, basis determines nodes

src/topology/quad4.jl

@@ -2,9 +2,16 @@
 # License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
 
 """
-    Quad4 <: AbstractTopology
+    Quadrilateral <: AbstractTopology
 
-Four-node quadrilateral element in 2D.
+Quadrilateral 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{Quadrilateral, 1}` → 4 nodes (Q1, bilinear)
+- `Serendipity{Quadrilateral, 2}` → 8 nodes (Q2, no center node)
+- `Lagrange{Quadrilateral, 2}` → 9 nodes (Q2, full tensor product)
+- `Lagrange{Quadrilateral, 3}` → 16 nodes (Q3, cubic)
 
 # Reference Element
 ```
@@ -20,45 +27,52 @@ Four-node quadrilateral element in 2D.
   1        2
 ```
 
-# Node Ordering
-Nodes are numbered counter-clockwise starting from (-1, -1):
+# Standard Node Positions
+For `Lagrange{Quadrilateral, 1}` (Q1, 4 nodes):
 1. (-1, -1) - Bottom-left
 2. ( 1, -1) - Bottom-right
 3. ( 1,  1) - Top-right
 4. (-1,  1) - Top-left
 
-# Properties
-- Nodes: 4
+For higher-order bases, additional nodes are added on edges, faces, and interior.
+
+# Topology Properties
 - Dimension: 2
 - Edges: 4
-- Faces: 1 (the element itself)
+- Faces: 1 (the element itself in 2D)
 
 # Typical Usage
 ```julia
-julia> topology = Quad4()
-julia> nnodes(topology)
-4
+julia> topology = Quadrilateral()
 julia> dim(topology)
 2
+julia> basis = Lagrange{Quadrilateral, 1}()
+julia> nnodes(basis)  # Node count comes from BASIS, not topology
+4
 ```
 
-See also: [`AbstractTopology`](@ref), [`Quad8`](@ref), [`Tri3`](@ref)
+See also: [`AbstractTopology`](@ref), [`Triangle`](@ref), [`Lagrange`](@ref), [`Serendipity`](@ref)
 """
-struct Quad4 <: AbstractTopology end
+struct Quadrilateral <: AbstractTopology end
 
-nnodes(::Quad4) = 4
-dim(::Quad4) = 2
+dim(::Quadrilateral) = 2
 
-function reference_coordinates(::Quad4)
+"""
+    reference_coordinates(::Quadrilateral)
+
+Get standard reference node positions for Quadrilateral (4 corners in parametric space).
+These are the Q1 (bilinear) node positions by default.
+"""
+function reference_coordinates(::Quadrilateral)
     return (
-        (-1.0, -1.0),  # Node 1
-        (1.0, -1.0),  # Node 2
-        (1.0, 1.0),  # Node 3
-        (-1.0, 1.0),  # Node 4
+        (-1.0, -1.0),  # Node 1: Bottom-left
+        (1.0, -1.0),  # Node 2: Bottom-right
+        (1.0, 1.0),  # Node 3: Top-right
+        (-1.0, 1.0),  # Node 4: Top-left
     )
 end
 
-function edges(::Quad4)
+function edges(::Quadrilateral)
     return (
         (1, 2),  # Edge 1: Bottom
         (2, 3),  # Edge 2: Right
@@ -68,4 +82,31 @@ function edges(::Quad4)
 end
 
 # For 2D elements, faces are the element itself
-faces(::Quad4) = ((1, 2, 3, 4),)
+faces(::Quadrilateral) = ((1, 2, 3, 4),)
+
+# ============================================================================
+# Deprecated aliases (for backwards compatibility)
+# ============================================================================
+
+"""
+    Quad4
+
+**DEPRECATED:** Use `Quadrilateral` + `Lagrange{Quadrilateral, 1}` instead.
+
+The old `Quad4` type conflated topology (quadrilateral) with node count (4).
+In the new architecture:
+- Topology defines geometric shape only
+- Basis functions determine node count
+
+Migration:
+```julia
+# Old (deprecated)
+element = Element(Quad4, (1,2,3,4))
+
+# New (correct)
+element = Element(Quadrilateral(), Lagrange{Quadrilateral, 1}(), Gauss{2}(), (1,2,3,4))
+```
+"""
+const Quad4 = Quadrilateral
+
+# Note: Quad4 is deprecated. Use Quadrilateral with parametric Lagrange basis instead.