Show resolved unit symbols in read-only Pset/Qto view; add write-back and fallback regression tests

Previously, unit symbols only appeared while a Pset/Qto was in edit mode
(pencil icon) -- the read-only summary view read raw {name: value} dicts
straight from ifcopenshell.util.element.get_psets(), a completely separate
path from the Attribute/unit_symbol machinery, so it never showed a label
even after the earlier fixes. This matters for the "someone in the field
just looking at values" use case, not just editing.

- bim/module/pset/data.py: switch to get_psets(verbose=True) to get each
  property's own entity id, then resolve its unit symbol the same
  override-aware way the edit-mode path does (tool.Pset.get_unit_symbol_for_prop).
  Falls back gracefully (empty symbol) for IfcPreDefinedPropertySet
  attributes, which aren't IfcProperty entities and can't carry a Unit
  override.
- bim/module/pset/ui.py: read-only value button now shows "250 mm" instead
  of just "250".

Also adds the regression tests planned but not yet committed:
- test/tool/test_pset.py: edit a property with its own Unit override and
  write it back, confirming no rescale and the override survives.
- test/bim/test_prop.py (new): get_display_name() falls back to the plain
  name (no crash) when no unit is resolvable or the project has no units
  assigned at all.

src/bonsai/test/tool/test_pset.py

@@ -20,6 +20,8 @@ import bpy
 import ifcopenshell
 import ifcopenshell.api
 import ifcopenshell.api.pset
+import ifcopenshell.api.root
+import ifcopenshell.api.unit
 
 import bonsai.core.tool
 import bonsai.tool as tool
@@ -52,3 +54,80 @@ class TestIsPsetEmpty(NewFile):
         assert subject.is_pset_empty(pset) is False
         ifcopenshell.api.pset.edit_pset(ifc, pset=pset, properties={"Foo": None})
         assert subject.is_pset_empty(pset) is True
+
+
+class TestEditingAnOverriddenUnitPropertyRoundTrips(NewFile):
+    def test_run(self):
+        # Project default is mm, but this property is authored directly in m.
+        ifc = ifcopenshell.file()
+        tool.Ifc.set(ifc)
+        ifcopenshell.api.root.create_entity(ifc, ifc_class="IfcProject")
+        length_mm = ifcopenshell.api.unit.add_si_unit(ifc, unit_type="LENGTHUNIT", prefix="MILLI")
+        ifcopenshell.api.unit.assign_unit(ifc, units=[length_mm])
+        length_m = ifcopenshell.api.unit.add_si_unit(ifc, unit_type="LENGTHUNIT")
+
+        element = ifc.createIfcWall()
+        pset = ifcopenshell.api.pset.add_pset(ifc, product=element, name="Pset_Test")
+        prop = ifc.createIfcPropertySingleValue(
+            Name="Foo", NominalValue=ifc.createIfcLengthMeasure(2.5), Unit=length_m
+        )
+        pset.HasProperties = [prop]
+
+        obj = bpy.data.objects.new("Wall", None)
+        tool.Ifc.link(element, obj)
+        blender_props = obj.PsetProperties
+        subject.import_pset_from_existing(pset, blender_props, None)
+
+        metadata = blender_props.properties["Foo"].metadata
+        assert metadata.unit_symbol == "m"
+        assert metadata.float_value == 2.5  # raw stored value, not rescaled to the project's mm
+
+        # Simulate a user edit in the property editor.
+        metadata.float_value = 3.5
+
+        # Simulate what EditPset.execute() does: collect the raw value straight
+        # off the metadata and write it back, with no rescaling step.
+        properties = {"Foo": metadata.get_value()}
+        ifcopenshell.api.pset.edit_pset(ifc, pset=pset, properties=properties)
+
+        assert prop.NominalValue.wrappedValue == 3.5  # not rescaled to 3500mm
+        assert prop.Unit == length_m  # override preserved
+
+
+class TestImportingATemplatedQuantityRespectsItsOwnUnitOverride(NewFile):
+    def test_run(self):
+        # Regression test: import_pset_from_template's Q_ branch used to
+        # unconditionally re-template existing quantities, which shadowed
+        # their own Unit override with the project default -- edit mode
+        # showed "m" while the read-only panel correctly showed "mm".
+        # Project default is m, but this quantity is authored directly in mm.
+        ifc = ifcopenshell.file()
+        tool.Ifc.set(ifc)
+        ifcopenshell.api.root.create_entity(ifc, ifc_class="IfcProject")
+        length_m = ifcopenshell.api.unit.add_si_unit(ifc, unit_type="LENGTHUNIT")
+        ifcopenshell.api.unit.assign_unit(ifc, units=[length_m])
+        length_mm = ifcopenshell.api.unit.add_si_unit(ifc, unit_type="LENGTHUNIT", prefix="MILLI")
+
+        element = ifc.createIfcBeam()
+        qto = ifcopenshell.api.pset.add_qto(ifc, product=element, name="Qto_Test")
+        quantity = ifc.createIfcQuantityLength(Name="Foo", Unit=length_mm, LengthValue=2500.0)
+        qto.Quantities = [quantity]
+
+        pset_template = ifc.createIfcPropertySetTemplate(
+            Name="Qto_Test",
+            TemplateType="PSET_TYPEDRIVENOVERRIDE",
+            ApplicableEntity="IfcBeam",
+            HasPropertyTemplates=[ifc.createIfcSimplePropertyTemplate(Name="Foo", TemplateType="Q_LENGTH")],
+        )
+
+        obj = bpy.data.objects.new("Beam", None)
+        tool.Ifc.link(element, obj)
+        blender_props = obj.PsetProperties
+        # Mirrors core/pset.py's enable_pset_editing: template pass, then existing-data pass.
+        subject.import_pset_from_template(pset_template, qto, blender_props)
+        subject.import_pset_from_existing(qto, blender_props, pset_template)
+
+        assert len(blender_props.properties) == 1  # not duplicated by the template pass
+        metadata = blender_props.properties["Foo"].metadata
+        assert metadata.unit_symbol == "mm"  # the quantity's own override, not the project default "m"
+        assert metadata.float_value == 2500.0  # raw stored value, not rescaled