feat(solver): implement OndselAdapter wrapping OndselSolver behind IKCSolver (#294)

Phase 1c of the pluggable solver system. Creates OndselAdapter — the
concrete IKCSolver implementation that wraps OndselSolver's Lagrangian
MBD engine behind the KCSolve abstraction layer.

The adapter translates KCSolve types (SolveContext, BaseJointKind,
Transform) to OndselSolver's ASMT hierarchy (ASMTAssembly, ASMTPart,
ASMTJoint, ASMTMarker) and extracts results back into SolveResult.
All 30+ OndselSolver #includes are confined to OndselAdapter.cpp.

Key implementation details:
- build_assembly(): creates ASMTAssembly from SolveContext
- create_joint(): maps 20 BaseJointKind values to ASMT joint types
  (eliminates the 35-case DistanceType switch — decomposition done upstream)
- Quaternion-to-rotation-matrix conversion for OndselSolver input
- Direct quaternion extraction for output (both use w,x,y,z convention)
- Drag lifecycle: pre_drag/drag_step/post_drag with stateful assembly
- Simulation lifecycle: run_kinematic/num_frames/update_for_frame
- Diagnostic extraction: iterates MBD system constraints for redundancy
- Static register_solver() for SolverRegistry integration
- ExternalSystem back-pointer NOT set — breaks bidirectional coupling

New files:
- Solver/OndselAdapter.h — class declaration with KCSolveExport
- Solver/OndselAdapter.cpp — full implementation (~530 lines)

Modified:
- Solver/CMakeLists.txt — add sources, link OndselSolver (PRIVATE)

src/Mod/Assembly/Solver/OndselAdapter.h

@@ -0,0 +1,128 @@
+// SPDX-License-Identifier: LGPL-2.1-or-later
+/****************************************************************************
+ *                                                                          *
+ *   Copyright (c) 2025 Kindred Systems <development@kindred-systems.com>   *
+ *                                                                          *
+ *   This file is part of FreeCAD.                                          *
+ *                                                                          *
+ *   FreeCAD is free software: you can redistribute it and/or modify it     *
+ *   under the terms of the GNU Lesser General Public License as            *
+ *   published by the Free Software Foundation, either version 2.1 of the   *
+ *   License, or (at your option) any later version.                        *
+ *                                                                          *
+ *   FreeCAD is distributed in the hope that it will be useful, but         *
+ *   WITHOUT ANY WARRANTY; without even the implied warranty of             *
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU       *
+ *   Lesser General Public License for more details.                        *
+ *                                                                          *
+ *   You should have received a copy of the GNU Lesser General Public       *
+ *   License along with FreeCAD. If not, see                                *
+ *   <https://www.gnu.org/licenses/>.                                       *
+ *                                                                          *
+ ***************************************************************************/
+
+#ifndef KCSOLVE_ONDSELADAPTER_H
+#define KCSOLVE_ONDSELADAPTER_H
+
+#include <memory>
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+#include "IKCSolver.h"
+#include "KCSolveGlobal.h"
+
+namespace MbD
+{
+class ASMTAssembly;
+class ASMTJoint;
+class ASMTMarker;
+class ASMTPart;
+}  // namespace MbD
+
+namespace KCSolve
+{
+
+/// IKCSolver implementation wrapping OndselSolver's Lagrangian MBD engine.
+///
+/// Translates KCSolve types (SolveContext, BaseJointKind, Transform) to
+/// OndselSolver's ASMT hierarchy (ASMTAssembly, ASMTPart, ASMTJoint, etc.)
+/// and extracts results back into SolveResult.
+///
+/// All OndselSolver #includes are confined to OndselAdapter.cpp.
+
+class KCSolveExport OndselAdapter : public IKCSolver
+{
+public:
+    OndselAdapter() = default;
+
+    // ── IKCSolver pure virtuals ────────────────────────────────────
+
+    std::string name() const override;
+    std::vector<BaseJointKind> supported_joints() const override;
+    SolveResult solve(const SolveContext& ctx) override;
+
+    // ── IKCSolver overrides ────────────────────────────────────────
+
+    SolveResult update(const SolveContext& ctx) override;
+
+    SolveResult pre_drag(const SolveContext& ctx,
+                         const std::vector<std::string>& drag_parts) override;
+    SolveResult drag_step(
+        const std::vector<SolveResult::PartResult>& drag_placements) override;
+    void post_drag() override;
+
+    SolveResult run_kinematic(const SolveContext& ctx) override;
+    std::size_t num_frames() const override;
+    SolveResult update_for_frame(std::size_t index) override;
+
+    std::vector<ConstraintDiagnostic> diagnose(const SolveContext& ctx) override;
+
+    bool is_deterministic() const override;
+    bool supports_bundle_fixed() const override;
+
+    /// Register OndselAdapter as "ondsel" in the SolverRegistry.
+    /// Call once at module init time.
+    static void register_solver();
+
+private:
+    // ── Assembly building ──────────────────────────────────────────
+
+    void build_assembly(const SolveContext& ctx);
+    std::shared_ptr<MbD::ASMTPart> make_part(const Part& part);
+    std::shared_ptr<MbD::ASMTMarker> make_marker(const std::string& name,
+                                                   const Transform& tf);
+    std::shared_ptr<MbD::ASMTJoint> create_joint(const Constraint& c);
+    void add_limits(const Constraint& c,
+                    const std::string& marker_i,
+                    const std::string& marker_j);
+    void add_motions(const SolveContext& ctx,
+                     const std::string& marker_i,
+                     const std::string& marker_j,
+                     const std::string& joint_id);
+    void fix_grounded_parts(const SolveContext& ctx);
+    void set_simulation_params(const SimulationParams& params);
+
+    // ── Result extraction ──────────────────────────────────────────
+
+    SolveResult extract_result() const;
+    std::vector<ConstraintDiagnostic> extract_diagnostics() const;
+    Transform extract_part_transform(
+        const std::shared_ptr<MbD::ASMTPart>& part) const;
+
+    // ── Quaternion ↔ rotation matrix conversion ────────────────────
+
+    /// Convert unit quaternion (w,x,y,z) to 3×3 rotation matrix (row-major).
+    static void quat_to_matrix(const std::array<double, 4>& q,
+                                double (&mat)[3][3]);
+
+    // ── Internal state ─────────────────────────────────────────────
+
+    std::shared_ptr<MbD::ASMTAssembly> assembly_;
+    std::unordered_map<std::string, std::shared_ptr<MbD::ASMTPart>> part_map_;
+    std::vector<std::string> drag_part_ids_;
+};
+
+}  // namespace KCSolve
+
+#endif  // KCSOLVE_ONDSELADAPTER_H