feat(solver): KCSolve solver addon with assembly integration (#289)

Adds the Kindred constraint solver as a pluggable Assembly workbench
backend, covering phases 3d through 5 of the solver roadmap.

Phase 3d: SolveContext packing
- Pack/unpack SolveContext into .kc archive on document save

Solver addon (mods/solver):
- Phase 1: Expression DAG, Newton-Raphson + BFGS, 3 basic constraints
- Phase 2: Full constraint vocabulary — all 24 BaseJointKind types
- Phase 3: Graph decomposition for cluster-by-cluster solving
- Phase 4: Per-entity DOF diagnostics, overconstrained detection,
  half-space preference tracking, minimum-movement weighting
- Phase 5: _build_system extraction, diagnose(), drag protocol,
  joint limits warning

Assembly workbench integration:
- Preference-driven solver selection (reads Mod/Assembly/Solver param)
- Solver backend combo box in Assembly preferences UI
- resetSolver() on AssemblyObject for live preference switching
- Integration tests (TestKindredSolverIntegration.py)
- In-client console test script (console_test_phase5.py)

src/Mod/Assembly/App/AssemblyObject.h

@@ -98,10 +98,15 @@ public:
     void postDrag();
     void savePlacementsForUndo();
     void undoSolve();
+    void resetSolver() { solver_.reset(); }
     void clearUndo();
 
     void exportAsASMT(std::string fileName);
 
+    /// Build the assembly constraint graph without solving.
+    /// Returns an empty SolveContext if no parts are grounded.
+    KCSolve::SolveContext getSolveContext();
+
     bool validateNewPlacements();
     void setNewPlacements();
     static void redrawJointPlacements(std::vector<App::DocumentObject*> joints);