Phase 8a MVP: CGAL traits + Discrete_conformal_map.h Euclidean entry

First step of the Phase 8 Hybrid MVP. Adds a thin CGAL-conformant public
API layer over the existing implementation, validated by 7 acceptance
tests.  Total CGAL test count: 183 (was 176), 0 skipped.

New public headers
──────────────────
* code/include/CGAL/Conformal_map_traits.h
    - ConformalMapTraits concept documentation
    - Default_conformal_map_traits<Surface_mesh<P>, K> specialisation
    - Static property-map accessors: vertex_points, theta_map,
      vertex_index_map, lambda0_map

* code/include/CGAL/Discrete_conformal_map.h
    - User-facing entry: discrete_conformal_map_euclidean(mesh, np)
    - Conformal_map_result<FT> struct (u, iter, ‖G‖, converged flags)
    - Natural-theta default: x = 0 is the equilibrium when no Θ supplied
    - Honours user-provided Θ via vertex_curvature_map named parameter

* code/include/CGAL/Conformal_map/internal/parameters.h
    - 4 named-parameter tags in CGAL::Conformal_map::internal_np:
        vertex_curvature_map, gradient_tolerance,
        max_iterations,       fixed_vertex_map
    - User-facing helpers in CGAL::parameters::*

Tests (test_cgal_traits_mvp.cpp, 7 cases)
─────────────────────────────────────────
* DefaultTraitsTypes:       compile-time type sanity (static_assert)
* AccessorsReuseExistingMaps: traits accessors return identical pmaps
* SingleTriangleConverges,
  QuadStripConverges:       end-to-end Euclidean wrapper passes
* MaxIterationsTakesEffect: named parameter is read
* GradientToleranceTakesEffect: tolerance override changes Newton end-state
* WrapperMatchesLegacyAPI:  cross-API result equality at 1e-10

Architecture
────────────
3-layer wrapper as designed (doc/api/cgal-package.md):
  Layer 1: code/include/*.hpp           (existing algorithms, unchanged)
  Layer 2: CGAL/Conformal_map/internal/ (adapter, parameter tags)
  Layer 3: CGAL/Conformal_map_traits.h, CGAL/Discrete_conformal_map.h
                                         (user-facing)

No algorithm duplication.  Existing 176 + 36 tests untouched.

Next: Phase 9a (Inversive-Distance) as the second client of this API —
the real acceptance test for the trait design.

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

code/include/CGAL/Conformal_map/internal/parameters.h

@@ -0,0 +1,127 @@
+// Copyright (c) 2024-2026 Tarik Moussa.
+// SPDX-License-Identifier: MIT
+//
+// Package: conformallab++ / Discrete_conformal_map (Phase 8 MVP, 2026-05-19)
+
+/*!
+\file CGAL/Conformal_map/internal/parameters.h
+\internal
+\ingroup PkgConformalMapRef
+
+Named-parameter tag definitions specific to the Discrete_conformal_map
+package.  These tags extend the CGAL named-parameter mechanism
+(see `<CGAL/Named_function_parameters.h>`).
+
+Usage from a user perspective is in `CGAL::parameters::*`; the tags
+themselves live in `CGAL::Conformal_map::internal_np`.
+
+This is an internal header — users should not include it directly.
+*/
+
+#ifndef CGAL_CONFORMAL_MAP_INTERNAL_PARAMETERS_H
+#define CGAL_CONFORMAL_MAP_INTERNAL_PARAMETERS_H
+
+#include <CGAL/Named_function_parameters.h>
+
+namespace CGAL {
+namespace Conformal_map {
+
+/// \internal
+/// Parameter tags for the conformal-map package.  Each tag is an
+/// `enum` whose name ends in `_t` and a value whose name does not.
+/// The pattern follows CGAL convention so that the existing
+/// `choose_parameter` / `get_parameter` machinery works directly.
+namespace internal_np {
+
+// ─── Target curvature (Θᵥ) ──────────────────────────────────────────────────
+/// Property-map: vertex_descriptor → FT (target cone angle Θᵥ in radians).
+/// Default: 2π at every interior vertex, π at every boundary vertex.
+enum vertex_curvature_map_t { vertex_curvature_map };
+
+// ─── Newton solver tolerances ───────────────────────────────────────────────
+/// Convergence threshold for the Newton solver: ‖G(u)‖∞ < tol.
+/// Type: FT.  Default: 1e-10.
+enum gradient_tolerance_t { gradient_tolerance };
+
+/// Maximum number of Newton iterations.
+/// Type: int.  Default: 200.
+/// (Reuses the CGAL `number_of_iterations` tag where appropriate; this
+/// alias is provided for vocabulary continuity within the package.)
+enum max_iterations_t { max_iterations };
+
+// ─── DOF / gauge fixing ─────────────────────────────────────────────────────
+/// Property-map: vertex_descriptor → bool.  `true` ⇒ vertex is pinned
+/// (u_v = 0, removed from the Newton DOF vector).
+/// Default: first vertex is pinned, all others are variable.
+enum fixed_vertex_map_t { fixed_vertex_map };
+
+}  // namespace internal_np
+}  // namespace Conformal_map
+
+namespace parameters {
+
+/*!
+\addtogroup PkgConformalMapNamedParameters
+\{
+*/
+
+/// \name Discrete conformal map — package-specific named parameters
+/// \{
+
+/// `vertex_curvature_map(pmap)` — target cone angle Θᵥ per vertex.
+/// Type: model of `ReadablePropertyMap` with key = `vertex_descriptor`,
+/// value = `FT`.  If omitted, the package uses 2π at interior vertices
+/// and π at boundary vertices (the natural Gauss–Bonnet target for an
+/// open disk or closed flat surface).
+template <typename PropertyMap>
+auto vertex_curvature_map(const PropertyMap& pmap)
+{
+    return CGAL::Named_function_parameters<
+        PropertyMap,
+        Conformal_map::internal_np::vertex_curvature_map_t,
+        CGAL::internal_np::No_property
+    >(pmap);
+}
+
+/// `gradient_tolerance(eps)` — Newton stopping criterion ‖G‖∞ < eps.
+template <typename FT>
+auto gradient_tolerance(FT eps)
+{
+    return CGAL::Named_function_parameters<
+        FT,
+        Conformal_map::internal_np::gradient_tolerance_t,
+        CGAL::internal_np::No_property
+    >(eps);
+}
+
+/// `max_iterations(n)` — Newton iteration limit.
+inline auto max_iterations(int n)
+{
+    return CGAL::Named_function_parameters<
+        int,
+        Conformal_map::internal_np::max_iterations_t,
+        CGAL::internal_np::No_property
+    >(n);
+}
+
+/// `fixed_vertex_map(pmap)` — which vertices are pinned for gauge-fixing.
+/// Type: model of `ReadablePropertyMap` with key = `vertex_descriptor`,
+/// value = `bool`.  If omitted, the first vertex in the mesh is pinned
+/// (compatible with the existing legacy API).
+template <typename PropertyMap>
+auto fixed_vertex_map(const PropertyMap& pmap)
+{
+    return CGAL::Named_function_parameters<
+        PropertyMap,
+        Conformal_map::internal_np::fixed_vertex_map_t,
+        CGAL::internal_np::No_property
+    >(pmap);
+}
+
+/// \}
+/// \}
+
+}  // namespace parameters
+}  // namespace CGAL
+
+#endif  // CGAL_CONFORMAL_MAP_INTERNAL_PARAMETERS_H