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/CommandCreateAssembly.py

@@ -22,15 +22,14 @@
 # **************************************************************************/
 
 import FreeCAD as App
-
 from PySide.QtCore import QT_TRANSLATE_NOOP
 
 if App.GuiUp:
     import FreeCADGui as Gui
     from PySide import QtCore, QtGui, QtWidgets
 
-import UtilsAssembly
 import Preferences
+import UtilsAssembly
 
 translate = App.Qt.translate
 
@@ -78,14 +77,22 @@ class CommandCreateAssembly:
                 'assembly = activeAssembly.newObject("Assembly::AssemblyObject", "Assembly")\n'
             )
         else:
-            commands = (
-                'assembly = App.ActiveDocument.addObject("Assembly::AssemblyObject", "Assembly")\n'
-            )
+            commands = 'assembly = App.ActiveDocument.addObject("Assembly::AssemblyObject", "Assembly")\n'
 
         commands = commands + 'assembly.Type = "Assembly"\n'
         commands = commands + 'assembly.newObject("Assembly::JointGroup", "Joints")'
 
         Gui.doCommand(commands)
+
+        # Make origin planes visible by default so they serve as
+        # reference geometry (like SolidWorks Front/Top/Right planes).
+        Gui.doCommandGui(
+            "assembly.Origin.ViewObject.Visibility = True\n"
+            "for feat in assembly.Origin.OriginFeatures:\n"
+            "    if feat.isDerivedFrom('App::Plane'):\n"
+            "        feat.ViewObject.Visibility = True\n"
+        )
+
         if not activeAssembly:
             Gui.doCommandGui("Gui.ActiveDocument.setEdit(assembly)")
 
@@ -98,7 +105,9 @@ class ActivateAssemblyTaskPanel:
     def __init__(self, assemblies):
         self.assemblies = assemblies
         self.form = QtWidgets.QWidget()
-        self.form.setWindowTitle(translate("Assembly_ActivateAssembly", "Activate Assembly"))
+        self.form.setWindowTitle(
+            translate("Assembly_ActivateAssembly", "Activate Assembly")
+        )
 
         layout = QtWidgets.QVBoxLayout(self.form)
         label = QtWidgets.QLabel(
@@ -132,9 +141,12 @@ class CommandActivateAssembly:
     def GetResources(self):
         return {
             "Pixmap": "Assembly_ActivateAssembly",
-            "MenuText": QT_TRANSLATE_NOOP("Assembly_ActivateAssembly", "Activate Assembly"),
+            "MenuText": QT_TRANSLATE_NOOP(
+                "Assembly_ActivateAssembly", "Activate Assembly"
+            ),
             "ToolTip": QT_TRANSLATE_NOOP(
-                "Assembly_ActivateAssembly", "Sets an assembly as the active one for editing."
+                "Assembly_ActivateAssembly",
+                "Sets an assembly as the active one for editing.",
             ),
             "CmdType": "ForEdit",
         }
@@ -156,7 +168,9 @@ class CommandActivateAssembly:
 
     def Activated(self):
         doc = App.ActiveDocument
-        assemblies = [o for o in doc.Objects if o.isDerivedFrom("Assembly::AssemblyObject")]
+        assemblies = [
+            o for o in doc.Objects if o.isDerivedFrom("Assembly::AssemblyObject")
+        ]
 
         if len(assemblies) == 1:
             # If there's only one, activate it directly without showing a dialog