properties: real attributes + relationships; keep selection on deselect

Add helpers to src/helpers/element: get_scalar_attributes (primitive
EXPRESS attributes only — entity refs / aggregates omitted), get_type
and get_container (ports of ifcopenshell.util.element), and a public
get_string_attribute for safe by-name reads.

Wire them into the properties panel:
- Attributes section shows the element's direct primitive attributes for
  live entities, or cached GlobalId / Name for geometry-only elements.
- Relationships section shows the construction Type and spatial Container
  by name (falling back to the class when unnamed).
- Placeholders are cleared once a project is loaded, so no mock data leaks.

Also: a deselect (click on empty space -> object_id 0) no longer resets
the panel; it keeps showing the last active object. Project reset/open
still clear it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/helpers/element.h

@@ -26,6 +26,8 @@
 
 #include <optional>
 #include <string>
+#include <utility>
+#include <vector>
 
 // Mirrors ifcopenshell.util.element.get_predefined_type. Returns the element's
 // PredefinedType, falling back to the user-defined ObjectType / ElementType /
@@ -35,4 +37,28 @@
 // IfcObject, or a geometry-only proxy with no live IFC data).
 std::optional<std::string> get_predefined_type(const express::Base& element);
 
+// Mirrors ifcopenshell.util.element.get_type: the construction type element of
+// an occurrence (via IsTypedBy on IFC4+, IsDefinedBy on IFC2X3). A type element
+// returns itself. Empty express::Base when the element is untyped.
+express::Base get_type(const express::Base& element);
+
+// Mirrors ifcopenshell.util.element.get_container (indirect, no ifc_class
+// filter): the spatial element that contains this element — the directly
+// containing spatial structure, or, for an aggregated part, the container of its
+// aggregate parent. Empty when uncontained. (The nest / filled-void /
+// voided-element branches of the Python original are not ported.)
+express::Base get_container(const express::Base& element);
+
+// Safely read a string- or enum-valued attribute by name (Python's getattr).
+// std::nullopt when the attribute is absent for this entity's type or IFC null.
+std::optional<std::string> get_string_attribute(const express::Base& element,
+                                                const std::string& name);
+
+// The element's direct EXPRESS attributes that have a primitive scalar value
+// (string / enum / integer / real / boolean / logical), as (name, formatted
+// value) pairs in declaration order. Attributes that are entity references,
+// aggregates / lists, or unset (IFC null) are omitted — so the caller gets a
+// flat, display-ready view with no nested objects.
+std::vector<std::pair<std::string, std::string>> get_scalar_attributes(const express::Base& element);
+
 #endif // ELEMENT_H