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).
