feat(phase5): Layout, CLI, JSON/XML serialisation — 95 tests

Phase 5 complete:

layout.hpp
  - euclidean_layout(): BFS unfolding in ℝ² using trilaterate_2d
  - spherical_layout(): BFS on S² using trilaterate_sph (spherical law of cosines)
  - hyper_ideal_layout(): BFS in Poincaré disk (tanh(d/2) Euclidean approx)
  - save_layout_off(): convenience OFF writer for 2-D and 3-D layouts

serialization.hpp
  - save/load_result_json(): nlohmann/json; stores DOF vector + uv/pos layout
  - save/load_result_xml(): hand-written writer/parser; same schema

conformallab_cli.cpp (rewritten)
  - CLI11 interface: -i/-o/-g/-j/-x/-s/-v
  - Dispatches to euclidean / spherical / hyper_ideal pipeline
  - Runs Newton, computes layout, saves OFF + JSON + XML

examples/example_layout.cpp
  - Full round-trip demo: solve → layout → JSON/XML → reload → verify

tests/cgal/test_layout.cpp (8 tests)
  - Euclidean_PreservesEdgeLengths, CorrectVertexCount, TriangleIsNonDegenerate
  - Spherical_PreservesArcLengths, PositionsOnUnitSphere
  - HyperIdeal_SuccessAndFinitePositions
  - Serialization.JSON_RoundTrip, XML_RoundTrip

All 95 CGAL tests pass (2 skipped — Hessian stubs unchanged).

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

code/tests/cgal/CMakeLists.txt

@@ -33,11 +33,15 @@ add_executable(conformallab_cgal_tests
 
     # ── Phase 4c: End-to-end pipeline + user examples ─────────────────────
     test_pipeline.cpp
+
+    # ── Phase 5: Layout / embedding + serialization ────────────────────────
+    test_layout.cpp
 )
 
 target_include_directories(conformallab_cgal_tests SYSTEM PRIVATE
     ${CMAKE_SOURCE_DIR}/deps/eigen-3.4.0
     ${CMAKE_SOURCE_DIR}/deps/CGAL-6.1.1/include
+    ${CMAKE_SOURCE_DIR}/deps/single_includes
     ${Boost_INCLUDE_DIRS}
 )