Add multi-select, specifier column, and visibility shortcuts to hierarchy panel

Ctrl+click toggles prim selection; Shift+click range-selects using a
per-frame visible-order list. Multi-selection is broadcast to the viewport
via a new SetOnPrimsSelected callback so Hydra highlights all selected prims.

Adds a 5th table column showing each prim's SdfSpecifier (def/over/class)
with colour coding. 'over' prims are shown in orange to make overrides
immediately visible.

Visibility icon updated: invisible prims show EyeSlash in orange instead
of grey. Ctrl+H / Shift+H keyboard shortcuts hide / show all selected prims.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui/Application.cpp

@@ -115,6 +115,11 @@ bool Application::Initialize(const std::string& windowTitle, int width, int heig
             m_curveEditorPanel->SetSelectedPrimPath(path);
         });
 
+    m_sceneHierarchyPanel->SetOnPrimsSelected(
+        [this](const std::vector<std::string>& paths) {
+            m_viewportPanel->SetSelectedPrimPaths(paths);
+        });
+
     m_sceneHierarchyPanel->SetOnStageMetadataChanged(
         [this]() {
             RefreshManagers();