feat(assembly): add diagnostic logging to solver and assembly

C++ (AssemblyObject):
- getOrCreateSolver: log which solver backend was loaded
- solve: log assembly name, grounded/joint counts, context size,
  result status with DOF and placement count, per-constraint
  diagnostics on failure
- preDrag/doDragStep/postDrag: log drag part count, per-step
  validation failures, and summary (total steps / rejected count)
- buildSolveContext: log grounded/free part counts, constraint count,
  limits count, and bundle_fixed flag

Python (kindred_solver submodule):
- solver.py: log solve entry/exit with timing, system build stats,
  decomposition decisions, Newton/BFGS fallback events, drag lifecycle
- decompose.py: log cluster stats and per-cluster convergence
- Init.py: FreeCAD log handler routing Python logging to Console

src/Mod/Assembly/App/AssemblyObject.h

@@ -280,6 +280,10 @@ private:
 
     bool bundleFixed;
 
+    // Drag diagnostic counters (reset in preDrag, reported in postDrag)
+    int dragStepCount_ = 0;
+    int dragStepRejected_ = 0;
+
     int lastDoF;
     bool lastHasConflict;
     bool lastHasRedundancies;