feat(phase4): HyperIdeal Newton solver, SparseQR fallback, examples, docs

Phase 4 complete — 87 CGAL tests pass, 2 skipped.

Newton solver (phase4a):
- hyper_ideal_hessian.hpp: symmetric FD Hessian (O(ε²), PSD by convexity)
- newton_hyper_ideal(): Newton + backtracking for the HyperIdeal functional
- detail::solve_with_fallback(): optional bool* fallback_used parameter
- solve_linear_system(): public API exposing LDLT→SparseQR fallback

SparseQR fallback tests (SparseQRFallback.*):
- FullRankSystem_CorrectSolution: LDLT path, fallback_used=false
- SingularMatrix_FallbackActivated: zero-pivot → QR activated, fallback_used=true
- Euclidean_ClosedMeshNoPinConverges: gauge-mode null space handled via QR

HyperIdeal Newton tests (NewtonSolver.HyperIdeal_*):
- ConvergesTriangleAllVariable, ResultFieldsConsistent,
  ConvergesTetrahedron, SparseQRFallbackNoCrash
- Natural-target base point (b=1.0, a=0.5) — x=0 is degenerate in log-space

Pipeline tests (test_pipeline.cpp):
- End-to-end: all three geometries, mesh I/O round-trip, solve+export

Example programs (code/examples/):
- example_euclidean.cpp:   headless Euclidean pipeline
- example_hyper_ideal.cpp: headless HyperIdeal pipeline
- example_viewer.cpp:      interactive libigl viewer with jet colour map

README:
- Mathematical scope table: C++ vs Java original (18 rows)
- "For mathematicians" section: mental model, step-by-step new-functional
  guide, half-edge traversal snippets, recommended reading

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

code/tests/cgal/CMakeLists.txt

@@ -30,6 +30,9 @@ add_executable(conformallab_cgal_tests
 
     # ── Phase 4b: Mesh I/O (CGAL::IO) ─────────────────────────────────────
     test_mesh_io.cpp
+
+    # ── Phase 4c: End-to-end pipeline + user examples ─────────────────────
+    test_pipeline.cpp
 )
 
 target_include_directories(conformallab_cgal_tests SYSTEM PRIVATE