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.cpp

@@ -176,6 +176,10 @@ KCSolve::IKCSolver* AssemblyObject::getOrCreateSolver()
         std::string solverName = hGrp->GetASCII("Solver", "");
         solver_ = KCSolve::SolverRegistry::instance().get(solverName);
         // get("") returns the registry default (first registered solver)
+        if (solver_) {
+            FC_LOG("Assembly : loaded solver '" << solver_->name()
+                   << "' (requested='" << solverName << "')");
+        }
     }
     return solver_.get();
 }
@@ -212,14 +216,22 @@ int AssemblyObject::solve(bool enableRedo, bool updateJCS)
 
     auto groundedObjs = getGroundedParts();
     if (groundedObjs.empty()) {
+        FC_LOG("Assembly : solve skipped — no grounded parts");
         return -6;
     }
 
     std::vector<App::DocumentObject*> joints = getJoints(updateJCS);
     removeUnconnectedJoints(joints, groundedObjs);
 
+    FC_LOG("Assembly : solve on '" << getFullLabel()
+           << "' — " << groundedObjs.size() << " grounded, "
+           << joints.size() << " joints");
+
     KCSolve::SolveContext ctx = buildSolveContext(joints);
 
+    FC_LOG("Assembly : solve context — " << ctx.parts.size() << " parts, "
+           << ctx.constraints.size() << " constraints");
+
     // Always save placements to enable orientation flip detection
     savePlacementsForUndo();
 
@@ -241,6 +253,13 @@ int AssemblyObject::solve(bool enableRedo, bool updateJCS)
     }
 
     if (lastResult_.status == KCSolve::SolveStatus::Failed) {
+        FC_LOG("Assembly : solve failed — status="
+               << static_cast<int>(lastResult_.status)
+               << ", " << lastResult_.diagnostics.size() << " diagnostics");
+        for (const auto& d : lastResult_.diagnostics) {
+            Base::Console().warning("Assembly : diagnostic [%s]: %s\n",
+                                    d.constraint_id.c_str(), d.detail.c_str());
+        }
         updateSolveStatus();
         return -1;
     }
@@ -248,6 +267,7 @@ int AssemblyObject::solve(bool enableRedo, bool updateJCS)
     // Validate that the solve didn't cause any parts to flip orientation
     if (!validateNewPlacements()) {
         // Restore previous placements - the solve found an invalid configuration
+        FC_LOG("Assembly : solve rejected — placement validation failed, undoing");
         undoSolve();
         lastSolverStatus = -2;
         updateSolveStatus();
@@ -265,6 +285,9 @@ int AssemblyObject::solve(bool enableRedo, bool updateJCS)
 
     updateSolveStatus();
 
+    FC_LOG("Assembly : solve succeeded — dof=" << lastResult_.dof
+           << ", " << lastResult_.placements.size() << " placements");
+
     return 0;
 }
 
@@ -409,6 +432,8 @@ size_t Assembly::AssemblyObject::numberOfFrames()
 void AssemblyObject::preDrag(std::vector<App::DocumentObject*> dragParts)
 {
     bundleFixed = true;
+    dragStepCount_ = 0;
+    dragStepRejected_ = 0;
 
     auto* solver = getOrCreateSolver();
     if (!solver) {
@@ -421,6 +446,7 @@ void AssemblyObject::preDrag(std::vector<App::DocumentObject*> dragParts)
 
     auto groundedObjs = getGroundedParts();
     if (groundedObjs.empty()) {
+        FC_LOG("Assembly : preDrag skipped — no grounded parts");
         bundleFixed = false;
         return;
     }
@@ -474,6 +500,10 @@ void AssemblyObject::preDrag(std::vector<App::DocumentObject*> dragParts)
         }
     }
 
+    FC_LOG("Assembly : preDrag — " << dragPartIds.size() << " drag part(s), "
+           << joints.size() << " joints, " << ctx.parts.size() << " parts, "
+           << ctx.constraints.size() << " constraints");
+
     savePlacementsForUndo();
 
     try {
@@ -482,11 +512,13 @@ void AssemblyObject::preDrag(std::vector<App::DocumentObject*> dragParts)
     }
     catch (...) {
         // If pre_drag fails, we still need to be in a valid state
+        FC_LOG("Assembly : preDrag — solver pre_drag threw exception");
     }
 }
 
 void AssemblyObject::doDragStep()
 {
+    dragStepCount_++;
     try {
         std::vector<KCSolve::SolveResult::PartResult> dragPlacements;
 
@@ -506,6 +538,10 @@ void AssemblyObject::doDragStep()
 
         lastResult_ = solver_->drag_step(dragPlacements);
 
+        if (lastResult_.status == KCSolve::SolveStatus::Failed) {
+            FC_LOG("Assembly : dragStep #" << dragStepCount_ << " — solver failed");
+        }
+
         if (validateNewPlacements()) {
             setNewPlacements();
 
@@ -517,9 +553,12 @@ void AssemblyObject::doDragStep()
                 }
             }
         }
+        else {
+            dragStepRejected_++;
+        }
     }
     catch (...) {
-        // We do nothing if a solve step fails.
+        FC_LOG("Assembly : dragStep #" << dragStepCount_ << " — exception");
     }
 }
 
@@ -631,6 +670,8 @@ bool AssemblyObject::validateNewPlacements()
 
 void AssemblyObject::postDrag()
 {
+    FC_LOG("Assembly : postDrag — " << dragStepCount_ << " steps, "
+           << dragStepRejected_ << " rejected");
     if (solver_) {
         solver_->post_drag();
     }
@@ -1342,6 +1383,23 @@ KCSolve::SolveContext AssemblyObject::buildSolveContext(
         ctx.simulation = sp;
     }
 
+    // Log context summary
+    {
+        int nGrounded = 0, nFree = 0, nLimits = 0;
+        for (const auto& p : ctx.parts) {
+            if (p.grounded) nGrounded++;
+            else nFree++;
+        }
+        for (const auto& c : ctx.constraints) {
+            if (!c.limits.empty()) nLimits++;
+        }
+        FC_LOG("Assembly : buildSolveContext — "
+               << nGrounded << " grounded + " << nFree << " free parts, "
+               << ctx.constraints.size() << " constraints"
+               << (nLimits ? (std::string(", ") + std::to_string(nLimits) + " with limits") : "")
+               << (ctx.bundle_fixed ? ", bundle_fixed=true" : ""));
+    }
+
     return ctx;
 }