Phase 9a-Newton: newton_cp_euclidean + newton_inversive_distance

Wires the two Phase-9a functionals into the Newton-solver layer so
they are operational end-to-end.  CGAL test count: 212 → 219 (+7).

Solvers
───────
* newton_cp_euclidean(mesh, x0, m, tol, max_iter)
    - Uses cp_euclidean_hessian — analytic 2×2-per-edge BPS-2010
      formula h_jk = sin θ / (cosh Δρ − cos θ).
    - SparseQR fallback handles the gauge-singular case when no face
      is pinned (caller error, but we recover gracefully).
    - Strictly-convex energy ⇒ quadratic convergence near optimum.

* newton_inversive_distance(mesh, x0, m, tol, max_iter, hess_eps)
    - Uses an inline FD Hessian (n × gradient evaluations per step) —
      mirrors the Phase 4a HyperIdeal solver in spirit.
    - Analytic alternative via Glickenstein 2011 eq. (4.6) is tracked
      in doc/roadmap/research-track.md as Phase 9a.2-analytic.
    - Sensitive to initial point; the test suite always starts from
      a natural-theta setup (u = 0 is the equilibrium when
      compute_inversive_distance_init_from_mesh was called).

Tests (test_newton_phase9a.cpp, 7 cases)
────────────────────────────────────────
* CPEuclidean_NaturalPhi_ClosedTetrahedron_ConvergesInZeroIterations
* CPEuclidean_PerturbedStart_ConvergesBackToEquilibrium
* CPEuclidean_OpenTetrahedron_NaturalPhi_Converges
* InversiveDistance_NaturalTheta_Triangle_ConvergesInZero
* InversiveDistance_PerturbedQuadStrip_Converges
* InversiveDistance_PerturbedTetrahedron_Converges
* CPEuclidean_UsesAnalyticHessian
    Regression guard: 3-DOF problem converges in ≤ 10 iterations even
    with strong perturbation, confirming the analytic Hessian path is
    actually used.

All seven tests pass.  Full CGAL suite: 219/219 PASSED, 0 SKIPPED.

Roadmap additions (`doc/roadmap/phases.md`)
───────────────────────────────────────────
New Phase 11+ section flags two Java sub-packages as optional/deferred
ports, recorded for project memory but not roadmap commitments:

* 11a — Schottky uniformisation (Java plugin/schottky/*, ~3000 LoC)
        Hyperbolic loxodromic group acting on S²; complement of the
        Phase 10c Fuchsian-group representation in H².  Requires
        Phase 10b period matrix + Möbius-group machinery from Phase 7.
        Effort: very large (4-6 weeks).

* 11b — Riemann maps (Java plugin/riemannmap/*, ~1500 LoC)
        Discrete Riemann mapping theorem; texture mapping of bounded
        planar regions, classical conformal mapping for engineering.
        Requires Phase 10b' quasi-isothermic or Phase 9a.1 CP-Euclidean.
        Effort: large (3-4 weeks).

Both are explicitly NOT roadmap commitments — they live in the doc so
they aren't re-discovered.

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

doc/roadmap/phases.md

@@ -245,3 +245,54 @@ Phase 10   Global uniformization for genus g ≥ 2
        None of these are required for the genus-g uniformization
        pipeline; they extend the breadth of methods.
 ```
+
+---
+
+## ◼ Phase 11+ — Specialised applications (optional, deferred)
+
+> **Status:** out-of-scope for v1.0 but recorded here so that future
+> contributors don't re-discover them.  Both items live in the Java
+> repo as plugin sub-packages and would benefit from porting *only*
+> after Phase 10 is complete (they require the period-matrix and
+> fundamental-domain infrastructure to be in place first).
+
+```
+11a   Schottky uniformisation
+        Java plugin:        plugin/schottky/* (~12 Java files, ~3 000 LoC)
+        Mathematical basis: Schottky group — discrete subgroup
+                            Γ ⊂ PSL(2,ℂ) generated by hyperbolic loxodromic
+                            elements, fundamental domain a sphere with
+                            2g disjoint discs removed.
+        Use case:           "handlebody" uniformisation, complement of
+                            Phase 10c's Fuchsian-group representation
+                            (Schottky represents Riemann surfaces as
+                            quotients of domains in S² rather than of H²).
+        Requires:           Phase 10b (period matrix) + working
+                            Möbius-group machinery from Phase 7.
+        Effort:             very large (4–6 weeks) — significant Java
+                            code, complex-analytic algorithms,
+                            substantial test design.
+
+11b   Riemann maps (planar conformal mapping)
+        Java plugin:        plugin/riemannmap/* (~6 Java files, ~1 500 LoC)
+        Mathematical basis: Riemann mapping theorem — every simply
+                            connected proper subdomain of ℂ is conformally
+                            equivalent to the unit disc.  Discrete version
+                            via circle packing or Schwarz-Christoffel-like
+                            formulae.
+        Use case:           Texture mapping of bounded planar regions;
+                            classical conformal mapping for engineering
+                            applications (electrostatics, fluid flow).
+        Requires:           Phase 10b' QuasiisothermicUtility or the
+                            CP-Euclidean machinery from Phase 9a.1
+                            (depending on the chosen discrete-Riemann
+                            algorithm).
+        Effort:             large (3–4 weeks) — smaller than Schottky
+                            but still substantial.  Heavy on
+                            visualisation; consider porting only the
+                            algorithmic core.
+
+Both items are tracked here so the project memory is preserved; they
+are NOT roadmap commitments.  See `research-track.md` for the formal
+research-versus-port classification before starting either.
+```