fix(assembly): move resetSolver() out-of-line to fix incomplete type error

unique_ptr::reset() requires the complete type for its deleter, but
IKCSolver is only forward-declared in AssemblyObject.h. Move the
definition to AssemblyObject.cpp where the full header is included.

src/Mod/Assembly/App/AssemblyObject.cpp

@@ -144,6 +144,11 @@ void AssemblyObject::onChanged(const App::Property* prop)
 
 // ── Solver integration ─────────────────────────────────────────────
 
+void AssemblyObject::resetSolver()
+{
+    solver_.reset();
+}
+
 KCSolve::IKCSolver* AssemblyObject::getOrCreateSolver()
 {
     if (!solver_) {

src/Mod/Assembly/App/AssemblyObject.h

@@ -98,7 +98,7 @@ public:
     void postDrag();
     void savePlacementsForUndo();
     void undoSolve();
-    void resetSolver() { solver_.reset(); }
+    void resetSolver();
     void clearUndo();
 
     void exportAsASMT(std::string fileName);