refactor(topology): Rename Seg2 to Segment, remove hardcoded node count

- Changed struct name from Seg2 to Segment
- Removed nnodes() method (node count now determined by basis)
- Updated all function signatures to use Segment instead of Seg2
- Added Seg2 as backwards compatibility alias
- Added note explaining basis determines node count

src/topology/seg2.jl

@@ -2,9 +2,9 @@
 # License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE
 
 """
-    Seg2 <: AbstractTopology
+    Segment <: AbstractTopology
 
-2-node linear segment/line element.
+Linear segment/line element topology (1D).
 
 Reference element in 1D parametric space [-1, 1].
 
@@ -14,36 +14,42 @@ Reference element in 1D parametric space [-1, 1].
 -1           +1
 ```
 
+**Note:** Node count determined by basis function degree:
+- Lagrange{Segment, 1}: 2 nodes (linear)
+- Lagrange{Segment, 2}: 3 nodes (quadratic)
+
 **Zero allocation:** All functions return compile-time sized tuples.
 """
-struct Seg2 <: AbstractTopology end
+struct Segment <: AbstractTopology end
 
-nnodes(::Seg2) = 2
-dim(::Seg2) = 1
+dim(::Segment) = 1
+
+# Backwards compatibility alias
+const Seg2 = Segment
 
 """
-    reference_coordinates(::Seg2) -> NTuple{2, NTuple{1, Float64}}
+    reference_coordinates(::Segment) -> NTuple{2, NTuple{1, Float64}}
 
-Reference coordinates for 2-node segment: (-1.0,) and (1.0,).
+Reference coordinates for segment endpoints: (-1.0,) and (1.0,).
 
 **Zero allocation:** Returns tuple of tuples (stack allocated).
 """
-reference_coordinates(::Seg2) = ((-1.0,), (1.0,))
+reference_coordinates(::Segment) = ((-1.0,), (1.0,))
 
 """
-    edges(::Seg2) -> NTuple{1, Tuple{Int, Int}}
+    edges(::Segment) -> NTuple{1, Tuple{Int, Int}}
 
 Edge connectivity for segment (the segment itself).
 
 **Zero allocation:** Returns tuple of tuples (stack allocated).
 """
-edges(::Seg2) = ((1, 2),)
+edges(::Segment) = ((1, 2),)
 
 """
-    faces(::Seg2) -> NTuple{0, Tuple{}}
+    faces(::Segment) -> NTuple{0, Tuple{}}
 
 Faces for 1D element (none in 1D).
 
 **Zero allocation:** Returns empty tuple (stack allocated).
 """
-faces(::Seg2) = ()
+faces(::Segment) = ()