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.

package.xml

@@ -0,0 +1,23 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<package format="1" xmlns="https://wiki.freecad.org/Package_Metadata">
+
+  <name>solver</name>
+  <description>Kindred constraint solver — expression-based Newton-Raphson</description>
+  <version>0.1.0</version>
+  <maintainer email="info@kindredsystems.io">Kindred Systems</maintainer>
+  <license file="LICENSE">LGPL-2.1-or-later</license>
+
+  <content>
+    <workbench>
+      <classname>KindredSolverBackend</classname>
+      <subdirectory>./</subdirectory>
+    </workbench>
+  </content>
+
+  <kindred>
+    <min_create_version>0.1.0</min_create_version>
+    <load_priority>10</load_priority>
+    <pure_python>true</pure_python>
+  </kindred>
+
+</package>