feat(assembly): fixed reference planes + solver docs

Assembly Origin Planes:
- AssemblyObject::setupObject() relabels origin planes to
  Top (XY), Front (XZ), Right (YZ) on assembly creation
- CommandCreateAssembly.py makes origin planes visible by default
- AssemblyUtils.cpp getObjFromRef() resolves LocalCoordinateSystem
  to child datum elements for joint references to origin planes
- TestAssemblyOriginPlanes.py: 9 integration tests covering
  structure, labels, grounding, reference resolution, solver,
  and save/load round-trip

Solver Documentation:
- docs/src/solver/: 7 new pages covering architecture overview,
  expression DAG, constraints, solving algorithms, diagnostics,
  assembly integration, and writing custom solvers
- docs/src/SUMMARY.md: added Kindred Solver section

src/Mod/Assembly/AssemblyTests/TestAssemblyOriginPlanes.py

@@ -0,0 +1,230 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+# /****************************************************************************
+#                                                                           *
+#    Copyright (c) 2025 Kindred Systems <development@kindred-systems.com>   *
+#                                                                           *
+#    This file is part of FreeCAD.                                          *
+#                                                                           *
+#    FreeCAD is free software: you can redistribute it and/or modify it     *
+#    under the terms of the GNU Lesser General Public License as            *
+#    published by the Free Software Foundation, either version 2.1 of the   *
+#    License, or (at your option) any later version.                        *
+#                                                                           *
+#    FreeCAD is distributed in the hope that it will be useful, but         *
+#    WITHOUT ANY WARRANTY; without even the implied warranty of             *
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU       *
+#    Lesser General Public License for more details.                        *
+#                                                                           *
+#    You should have received a copy of the GNU Lesser General Public       *
+#    License along with FreeCAD. If not, see                                *
+#    <https://www.gnu.org/licenses/>.                                       *
+#                                                                           *
+# ***************************************************************************/
+
+"""
+Tests for assembly origin reference planes.
+
+Verifies that new assemblies have properly labeled, grounded origin planes
+and that joints can reference them for solving.
+"""
+
+import os
+import tempfile
+import unittest
+
+import FreeCAD as App
+import JointObject
+import UtilsAssembly
+
+
+class TestAssemblyOriginPlanes(unittest.TestCase):
+    """Tests for assembly origin planes (Top/Front/Right)."""
+
+    def setUp(self):
+        doc_name = self.__class__.__name__
+        if App.ActiveDocument:
+            if App.ActiveDocument.Name != doc_name:
+                App.newDocument(doc_name)
+        else:
+            App.newDocument(doc_name)
+        App.setActiveDocument(doc_name)
+        self.doc = App.ActiveDocument
+
+        self.assembly = self.doc.addObject("Assembly::AssemblyObject", "Assembly")
+        self.jointgroup = self.assembly.newObject("Assembly::JointGroup", "Joints")
+
+    def tearDown(self):
+        App.closeDocument(self.doc.Name)
+
+    # ── Helpers ─────────────────────────────────────────────────────
+
+    def _get_origin(self):
+        return self.assembly.Origin
+
+    def _make_box(self, x=0, y=0, z=0, size=10):
+        box = self.assembly.newObject("Part::Box", "Box")
+        box.Length = size
+        box.Width = size
+        box.Height = size
+        box.Placement = App.Placement(App.Vector(x, y, z), App.Rotation())
+        return box
+
+    def _make_joint(self, joint_type, ref1, ref2):
+        joint = self.jointgroup.newObject("App::FeaturePython", "Joint")
+        JointObject.Joint(joint, joint_type)
+        refs = [
+            [ref1[0], ref1[1]],
+            [ref2[0], ref2[1]],
+        ]
+        joint.Proxy.setJointConnectors(joint, refs)
+        return joint
+
+    # ── Structure tests ─────────────────────────────────────────────
+
+    def test_assembly_has_origin(self):
+        """New assembly has an Origin with 3 planes, 3 axes, 1 point."""
+        origin = self._get_origin()
+        self.assertIsNotNone(origin)
+        self.assertTrue(origin.isDerivedFrom("App::LocalCoordinateSystem"))
+
+        planes = origin.planes()
+        self.assertEqual(len(planes), 3)
+
+        axes = origin.axes()
+        self.assertEqual(len(axes), 3)
+
+    def test_origin_planes_labeled(self):
+        """Origin planes are labeled Top, Front, Right."""
+        origin = self._get_origin()
+
+        xy = origin.getXY()
+        xz = origin.getXZ()
+        yz = origin.getYZ()
+
+        self.assertEqual(xy.Label, "Top")
+        self.assertEqual(xz.Label, "Front")
+        self.assertEqual(yz.Label, "Right")
+
+    def test_origin_planes_have_correct_roles(self):
+        """Origin planes retain correct internal Role names."""
+        origin = self._get_origin()
+
+        self.assertEqual(origin.getXY().Role, "XY_Plane")
+        self.assertEqual(origin.getXZ().Role, "XZ_Plane")
+        self.assertEqual(origin.getYZ().Role, "YZ_Plane")
+
+    # ── Grounding tests ─────────────────────────────────────────────
+
+    def test_origin_in_grounded_set(self):
+        """Origin is part of the assembly's grounded set."""
+        grounded = self.assembly.getGroundedParts()
+        origin = self._get_origin()
+
+        grounded_names = {obj.Name for obj in grounded}
+        self.assertIn(origin.Name, grounded_names)
+
+    # ── Reference resolution tests ──────────────────────────────────
+
+    def test_getObject_resolves_origin_plane(self):
+        """UtilsAssembly.getObject correctly resolves an origin plane ref."""
+        origin = self._get_origin()
+        xy = origin.getXY()
+
+        # Ref structure: [Origin, ["XY_Plane.", "XY_Plane."]]
+        ref = [origin, [xy.Name + ".", xy.Name + "."]]
+        obj = UtilsAssembly.getObject(ref)
+        self.assertEqual(obj, xy)
+
+    def test_findPlacement_origin_plane_returns_identity(self):
+        """findPlacement for an origin plane (whole-object) returns identity."""
+        origin = self._get_origin()
+        xy = origin.getXY()
+
+        ref = [origin, [xy.Name + ".", xy.Name + "."]]
+        plc = UtilsAssembly.findPlacement(ref)
+
+        # For datum planes with no element, identity is returned.
+        # The actual orientation comes from the solver's getGlobalPlacement.
+        self.assertTrue(
+            plc.isSame(App.Placement(), 1e-6),
+            "findPlacement for origin plane should return identity",
+        )
+
+    # ── Joint / solve tests ─────────────────────────────────────────
+
+    def test_fixed_joint_to_origin_plane(self):
+        """Fixed joint referencing an origin plane solves correctly."""
+        origin = self._get_origin()
+        xy = origin.getXY()
+
+        box = self._make_box(50, 50, 50)
+
+        # Fixed joint (type 0): origin XY plane ↔ box Face1 (bottom, Z=0)
+        self._make_joint(
+            0,
+            [origin, [xy.Name + ".", xy.Name + "."]],
+            [box, ["Face1", "Vertex1"]],
+        )
+
+        # After solve, the box should have moved so that its Face1 (bottom)
+        # aligns with the XY plane (Z=0). The box bottom vertex1 is at (0,0,0).
+        self.assertAlmostEqual(
+            box.Placement.Base.z,
+            0.0,
+            places=3,
+            msg="Box should be on XY plane after fixed joint to Top plane",
+        )
+
+    def test_solve_return_code_with_origin_plane(self):
+        """Solve with an origin plane joint returns success (0)."""
+        origin = self._get_origin()
+        xz = origin.getXZ()
+
+        box = self._make_box(0, 100, 0)
+
+        self._make_joint(
+            0,
+            [origin, [xz.Name + ".", xz.Name + "."]],
+            [box, ["Face1", "Vertex1"]],
+        )
+
+        result = self.assembly.solve()
+        self.assertEqual(result, 0, "Solve should succeed with origin plane joint")
+
+    # ── Round-trip test ──────────────────────────────────────────────
+
+    def test_save_load_preserves_labels(self):
+        """Labels survive save/load round-trip."""
+        origin = self._get_origin()
+
+        # Verify labels before save
+        self.assertEqual(origin.getXY().Label, "Top")
+        self.assertEqual(origin.getXZ().Label, "Front")
+        self.assertEqual(origin.getYZ().Label, "Right")
+
+        # Save to temp file
+        tmp = tempfile.mktemp(suffix=".FCStd")
+        try:
+            self.doc.saveAs(tmp)
+
+            # Close and reopen
+            doc_name = self.doc.Name
+            App.closeDocument(doc_name)
+            App.openDocument(tmp)
+            doc = App.ActiveDocument
+
+            assembly = doc.getObject("Assembly")
+            self.assertIsNotNone(assembly)
+
+            origin = assembly.Origin
+            self.assertEqual(origin.getXY().Label, "Top")
+            self.assertEqual(origin.getXZ().Label, "Front")
+            self.assertEqual(origin.getYZ().Label, "Right")
+
+            App.closeDocument(doc.Name)
+        finally:
+            if os.path.exists(tmp):
+                os.remove(tmp)
+
+        # Reopen a fresh doc for tearDown
+        App.newDocument(self.__class__.__name__)