feat(phase3a): introduce CGAL Surface_mesh as ConformalMesh foundation

Replaces the Java CoHDS with CGAL::Surface_mesh<Point3> (Simple_cartesian
kernel). Adds domain-specific property maps for lambda/theta/idx/alpha and
face geometry type — the direct C++ equivalent of CoVertex/CoEdge adapters.

New files:
  include/conformal_mesh.hpp   — ConformalMesh type + property-map helpers
  include/mesh_builder.hpp     — mesh factories (triangle, tetrahedron,
                                 quad-strip, fan) for tests and examples
  tests/cgal/                  — second test executable (conformallab_cgal_tests)
                                 built only with -DWITH_CGAL=ON

Test results (local, -DWITH_CGAL=ON):
  conformallab_tests:      36 registered | 23 passed | 13 skipped | 0 failed
  conformallab_cgal_tests: 14 registered | 14 passed |  0 skipped | 0 failed

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

code/include/mesh_builder.hpp

@@ -0,0 +1,109 @@
+#pragma once
+// mesh_builder.hpp
+//
+// Factory functions that build simple reference meshes for testing and examples.
+// All functions return a ConformalMesh (CGAL::Surface_mesh<Point3>).
+//
+// Replaces Java mesh generators:
+//   CoHDS generators (convex hull, hyper-ideal generator) come later (Phase 3c/4).
+//   These builders cover the minimal meshes needed for functional unit tests.
+
+#include "conformal_mesh.hpp"
+#include <cmath>
+#include <vector>
+
+namespace conformallab {
+
+// ── Single triangle ──────────────────────────────────────────────────────────
+//
+//   v2
+//   |  \
+//   |    \
+//   v0 ─ v1
+//
+// Returns a mesh with 1 face, 3 vertices, 3 edges.
+// The triangle lies in the xy-plane with a right angle at v0.
+inline ConformalMesh make_triangle(
+    double x0=0, double y0=0,
+    double x1=1, double y1=0,
+    double x2=0, double y2=1)
+{
+    ConformalMesh mesh;
+    auto v0 = mesh.add_vertex(Point3(x0, y0, 0));
+    auto v1 = mesh.add_vertex(Point3(x1, y1, 0));
+    auto v2 = mesh.add_vertex(Point3(x2, y2, 0));
+    mesh.add_face(v0, v1, v2);
+    return mesh;
+}
+
+// ── Regular tetrahedron ──────────────────────────────────────────────────────
+//
+// 4 vertices, 4 faces, 6 edges.
+// Euler characteristic: V - E + F = 4 - 6 + 4 = 2  (sphere topology).
+// Used to test closed-surface traversal.
+inline ConformalMesh make_tetrahedron()
+{
+    ConformalMesh mesh;
+
+    // Vertices of a regular tetrahedron centred at origin, edge length √2·2
+    auto v0 = mesh.add_vertex(Point3( 1,  1,  1));
+    auto v1 = mesh.add_vertex(Point3( 1, -1, -1));
+    auto v2 = mesh.add_vertex(Point3(-1,  1, -1));
+    auto v3 = mesh.add_vertex(Point3(-1, -1,  1));
+
+    // 4 outward-facing triangles (consistent winding)
+    mesh.add_face(v0, v2, v1);  // bottom (z=-1 side)
+    mesh.add_face(v0, v1, v3);  // front  (y=-1 side)
+    mesh.add_face(v0, v3, v2);  // left   (x=-1 side)
+    mesh.add_face(v1, v2, v3);  // back
+
+    return mesh;
+}
+
+// ── Two-triangle strip ───────────────────────────────────────────────────────
+//
+//   v2 ─ v3
+//   |  \ |
+//   v0 ─ v1
+//
+// 4 vertices, 2 faces, 5 edges (1 interior edge v1–v2 shared by both faces).
+// Useful for testing edge-interior vs edge-boundary distinction.
+inline ConformalMesh make_quad_strip()
+{
+    ConformalMesh mesh;
+    auto v0 = mesh.add_vertex(Point3(0, 0, 0));
+    auto v1 = mesh.add_vertex(Point3(1, 0, 0));
+    auto v2 = mesh.add_vertex(Point3(0, 1, 0));
+    auto v3 = mesh.add_vertex(Point3(1, 1, 0));
+
+    mesh.add_face(v0, v1, v2);   // lower-left  triangle
+    mesh.add_face(v1, v3, v2);   // upper-right triangle  (shares edge v1–v2)
+
+    return mesh;
+}
+
+// ── Regular flat polygon fan ─────────────────────────────────────────────────
+//
+// n triangles sharing a central vertex; forms a disk topology (boundary).
+// Used to verify valence-n vertex traversal.
+inline ConformalMesh make_fan(int n)
+{
+    CGAL_precondition(n >= 3);
+    ConformalMesh mesh;
+
+    auto center = mesh.add_vertex(Point3(0, 0, 0));
+
+    const double dtheta = 2.0 * M_PI / n;
+    std::vector<Vertex_index> rim(n);
+    for (int i = 0; i < n; ++i) {
+        double a = i * dtheta;
+        rim[i] = mesh.add_vertex(Point3(std::cos(a), std::sin(a), 0));
+    }
+
+    for (int i = 0; i < n; ++i)
+        mesh.add_face(center, rim[i], rim[(i+1) % n]);
+
+    return mesh;
+}
+
+} // namespace conformallab