feat(solver): implement SolverRegistry with plugin loading (#293)

Phase 1b of the pluggable solver system. Converts KCSolve from a
header-only INTERFACE target to a SHARED library and implements
the SolverRegistry with dynamic plugin discovery.

Changes:
- Add KCSolveGlobal.h export macro header (KCSolveExport)
- Move SolverRegistry method bodies from header to SolverRegistry.cpp
- Implement scan() with dlopen/LoadLibrary plugin loading
- Add scan_default_paths() for KCSOLVE_PLUGIN_PATH + system paths
- Plugin entry points: kcsolve_api_version() + kcsolve_create()
- API version checking (major version compatibility)
- Convert CMakeLists.txt from INTERFACE to SHARED library
- Link FreeCADBase (PRIVATE) for Console logging
- Link dl on POSIX for dynamic loading
- Fix -Wmissing-field-initializers warnings in IKCSolver.h defaults

The registry discovers plugins by scanning directories for shared
libraries that export the kcsolve C entry points. Plugins are
validated for API version compatibility before registration.
Manual registration via register_solver() remains available for
built-in solvers (e.g. OndselAdapter in Phase 1c).

src/Mod/Assembly/Solver/KCSolveGlobal.h

@@ -0,0 +1,37 @@
+// 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/>.                                       *
+ *                                                                          *
+ ***************************************************************************/
+
+#include <FCGlobal.h>
+
+#ifndef KCSOLVE_GLOBAL_H
+#define KCSOLVE_GLOBAL_H
+
+#ifndef KCSolveExport
+# ifdef KCSolve_EXPORTS
+#  define KCSolveExport FREECAD_DECL_EXPORT
+# else
+#  define KCSolveExport FREECAD_DECL_IMPORT
+# endif
+#endif
+
+#endif  // KCSOLVE_GLOBAL_H