feat(solver): define IKCSolver C++ API types and interface (#292)

Add the pluggable solver API as header-only files under
src/Mod/Assembly/Solver/. This is Phase 1a of the pluggable solver
system (INTER_SOLVER.md).

New files:
- Types.h: BaseJointKind enum (24 decomposed constraint types),
  Transform, Part, Constraint, SolveContext, SolveResult, and
  supporting types. Uses standalone types (no FreeCAD dependency)
  for future server worker compatibility.
- IKCSolver.h: Abstract solver interface with solve(), drag protocol
  (pre_drag/drag_step/post_drag), kinematic simulation
  (run_kinematic/num_frames/update_for_frame), and diagnostics.
  Only solve(), name(), and supported_joints() are pure virtual;
  all other methods have default implementations.
- SolverRegistry.h: Thread-safe singleton registry for solver
  backends with factory-based registration and default solver
  selection.
- CMakeLists.txt: INTERFACE library target (header-only for now).

Modified:
- Assembly/CMakeLists.txt: add_subdirectory(Solver)
- Assembly/App/CMakeLists.txt: link KCSolve INTERFACE target

src/Mod/Assembly/App/CMakeLists.txt

@@ -6,6 +6,7 @@ set(Assembly_LIBS
     Spreadsheet
     FreeCADApp
     OndselSolver
+    KCSolve
 )
 
 generate_from_py(AssemblyObject)