feat: add Phase 1 constraint solver addon, move prior content to GNN/

- Move existing OndselSolver, GNN ML layer, and tooling into GNN/
  directory for integration in later phases
- Add Create addon scaffold: package.xml, Init.py
- Add expression DAG with eval, symbolic diff, simplification
- Add parameter table with fixed/free variable tracking
- Add quaternion rotation as polynomial Expr trees
- Add RigidBody entity (7 DOF: position + unit quaternion)
- Add constraint classes: Coincident, DistancePointPoint, Fixed
- Add Newton-Raphson solver with symbolic Jacobian + numpy lstsq
- Add pre-solve passes: substitution + single-equation
- Add DOF counting via Jacobian SVD rank
- Add KindredSolver IKCSolver bridge for kcsolve integration
- Add 82 unit tests covering all modules

Registers as 'kindred' solver via kcsolve.register_solver() when
loaded by Create's addon_loader.

Init.py

@@ -0,0 +1,12 @@
+"""Register the Kindred solver with the KCSolve solver registry."""
+
+import FreeCAD
+
+try:
+    import kcsolve
+    from kindred_solver import KindredSolver
+
+    kcsolve.register_solver("kindred", KindredSolver)
+    FreeCAD.Console.PrintLog("kindred-solver registered\n")
+except Exception as exc:
+    FreeCAD.Console.PrintWarning(f"kindred-solver: registration failed: {exc}\n")