Fix loading project library without IfcProject

Per IFC4+, IfcContext is the abstract supertype of IfcProject and
IfcProjectLibrary; library-only files legitimately contain only
IfcProjectLibrary as their root context. Bonsai assumed an IfcProject
was always present at three crash sites: the parent-library enum
(reported in #8183), RefreshLibrary's tree view, and AddProjectLibrary.

Introduce tool.Project.get_root_context() that prefers IfcProject and
falls back to IfcProjectLibrary, and route the three sites through it.
get_parent_library() now returns None for a root IfcProjectLibrary;
get_project_hierarchy() and the EditProjectLibrary parent-swap branch
handle that. AddProjectLibrary creates the nested sub-library via
IfcRelNests when the root is an IfcProjectLibrary, matching the
existing convention for library-under-library nesting.

For the separate "Open IFC Project" path, abort with a friendly error
pointing users to Project Setup -> Project Library -> Select Library
File instead of letting set_units() crash deep in the importer.

Closes #8183.

Partly generated with the assistance of an AI coding tool.

src/bonsai/test/bim/module/project/test_project_library_data.py

@@ -0,0 +1,121 @@
+# Bonsai - OpenBIM Blender Add-on
+# Copyright (C) 2026
+#
+# This file is part of Bonsai.
+#
+# Bonsai is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# Bonsai 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 General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with Bonsai.  If not, see <http://www.gnu.org/licenses/>.
+#
+# This file was generated with the assistance of an AI coding tool.
+
+import ifcopenshell
+import ifcopenshell.api.nest
+import ifcopenshell.api.project
+import ifcopenshell.api.root
+import pytest
+
+import bonsai.tool as tool
+from bonsai.bim.ifc import IfcStore
+from bonsai.bim.module.project.data import ProjectLibraryData
+from test.bim.bootstrap import NewIfc
+
+pytestmark = pytest.mark.project
+
+
+def _make_library_only_file(*, with_child: bool = False) -> ifcopenshell.file:
+    """Build a minimal IFC4 file containing only an IfcProjectLibrary (no IfcProject).
+
+    Per IFC4+, a file must contain at least one IfcContext; IfcProjectLibrary is a
+    valid root on its own. ``with_child=True`` nests a sub-library under the root via
+    IfcRelNests, mirroring real authored library files.
+    """
+    library_file = ifcopenshell.api.project.create_file(version="IFC4")
+    root = ifcopenshell.api.root.create_entity(library_file, ifc_class="IfcProjectLibrary", name="RootLib")
+    if with_child:
+        child = ifcopenshell.api.root.create_entity(library_file, ifc_class="IfcProjectLibrary", name="ChildLib")
+        ifcopenshell.api.nest.assign_object(library_file, [child], root)
+    return library_file
+
+
+class TestLibraryOnlyFile(NewIfc):
+    def test_get_root_context_returns_project_library_when_no_project(self):
+        library_file = _make_library_only_file()
+        assert not library_file.by_type("IfcProject")
+
+        root = tool.Project.get_root_context(library_file)
+
+        assert root.is_a("IfcProjectLibrary")
+        assert root.Name == "RootLib"
+
+    def test_get_parent_library_returns_none_for_root_library(self):
+        library_file = _make_library_only_file()
+        root = library_file.by_type("IfcProjectLibrary")[0]
+
+        assert tool.Project.get_parent_library(root) is None
+
+    def test_get_project_hierarchy_skips_root_library(self):
+        library_file = _make_library_only_file(with_child=True)
+        root = next(lib for lib in library_file.by_type("IfcProjectLibrary") if lib.Name == "RootLib")
+        child = next(lib for lib in library_file.by_type("IfcProjectLibrary") if lib.Name == "ChildLib")
+
+        hierarchy = tool.Project.get_project_hierarchy(library_file)
+
+        assert root in hierarchy
+        assert child in hierarchy[root]
+
+    def test_project_library_data_loads_without_crash(self):
+        IfcStore.library_file = _make_library_only_file()
+        try:
+            ProjectLibraryData.is_loaded = False
+            ProjectLibraryData.load()
+            assert ProjectLibraryData.is_loaded
+            enum = ProjectLibraryData.data["parent_libraries_enum"]
+            assert len(enum) == 1
+            assert enum[0][1].startswith("IfcProjectLibrary ")
+        finally:
+            IfcStore.library_file = None
+            ProjectLibraryData.is_loaded = False
+
+    def test_refresh_library_succeeds_on_library_only_file(self):
+        import bpy
+
+        IfcStore.library_file = _make_library_only_file(with_child=True)
+        try:
+            result = bpy.ops.bim.refresh_library()
+            assert result == {"FINISHED"}
+        finally:
+            IfcStore.library_file = None
+            ProjectLibraryData.is_loaded = False
+
+    def test_add_project_library_nests_under_root_when_no_project(self):
+        import bpy
+
+        IfcStore.library_file = _make_library_only_file()
+        library_file = IfcStore.library_file
+        try:
+            root = library_file.by_type("IfcProjectLibrary")[0]
+            before = set(library_file.by_type("IfcProjectLibrary"))
+
+            result = bpy.ops.bim.add_project_library()
+
+            assert result == {"FINISHED"}
+            after = set(library_file.by_type("IfcProjectLibrary"))
+            new_libraries = after - before
+            assert len(new_libraries) == 1
+            new_library = next(iter(new_libraries))
+            assert new_library.Nests
+            assert new_library.Nests[0].RelatingObject == root
+            assert not new_library.HasContext
+        finally:
+            IfcStore.library_file = None
+            ProjectLibraryData.is_loaded = False