feat(examples): Add Gmsh geometry file for unit square mesh

New file: examples/gmsh_heat_equation/unit_square.geo (32 lines)

Defines unit square [0,1]×[0,1] with:
- 4 corner points with mesh size lc=0.1
- 4 boundary edges (bottom, right, top, left)
- Plane surface for 2D heat equation
- Physical groups labeled for boundary conditions
- Triangular elements (Tri3, ElementOrder=1)
- Frontal-Delaunay meshing algorithm

Generate mesh with: gmsh -2 unit_square.geo -o unit_square.msh

examples/gmsh_heat_equation/unit_square.geo

@@ -0,0 +1,32 @@
+// Gmsh geometry file: Unit square mesh for heat equation tutorial
+// Generate with: gmsh -2 unit_square.geo -o unit_square.msh
+
+// Mesh element size
+lc = 0.1;
+
+// Corner points
+Point(1) = {0, 0, 0, lc};
+Point(2) = {1, 0, 0, lc};
+Point(3) = {1, 1, 0, lc};
+Point(4) = {0, 1, 0, lc};
+
+// Edges
+Line(1) = {1, 2};  // Bottom
+Line(2) = {2, 3};  // Right
+Line(3) = {3, 4};  // Top
+Line(4) = {4, 1};  // Left
+
+// Surface
+Line Loop(1) = {1, 2, 3, 4};
+Plane Surface(1) = {1};
+
+// Physical groups for boundary conditions
+Physical Line("bottom") = {1};
+Physical Line("right") = {2};
+Physical Line("top") = {3};
+Physical Line("left") = {4};
+Physical Surface("body") = {1};
+
+// Use triangular elements
+Mesh.ElementOrder = 1;  // Linear elements (Tri3)
+Mesh.Algorithm = 6;     // Frontal-Delaunay for 2D