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/SceneHierarchyPanel.h

@@ -1,4 +1,4 @@
-#pragma once
+#pragma once
 
 #include "../core/PropertyManager.h"
 #include "../core/CommandHistory.h"
@@ -62,6 +62,9 @@ public:
     using PrimSelectCallback = std::function<void(const std::string& path)>;
     void SetOnPrimSelected(PrimSelectCallback callback) { m_onPrimSelected = callback; }
 
+    using MultiPrimSelectCallback = std::function<void(const std::vector<std::string>&)>;
+    void SetOnPrimsSelected(MultiPrimSelectCallback cb) { m_onPrimsSelected = cb; }
+
     /// Called when stage-level metadata (e.g. up axis) is changed via the hierarchy panel.
     using StageMetadataChangedCallback = std::function<void()>;
     void SetOnStageMetadataChanged(StageMetadataChangedCallback callback) { m_onStageMetadataChanged = callback; }
@@ -96,7 +99,12 @@ private:
     /// NOT layers that came in through references or payloads.
     std::unordered_set<std::string> m_localLayers;
 
+    /// Visible prim order rebuilt each frame — used for Shift+RMB range select.
+    std::vector<pxr::SdfPath>        m_visiblePrimOrder;
+    pxr::SdfPath                     m_rangeAnchorPath;
+
     PrimSelectCallback           m_onPrimSelected;
+    MultiPrimSelectCallback      m_onPrimsSelected;
     StageMetadataChangedCallback m_onStageMetadataChanged;
 
     /// Remove-prim confirmation state.
@@ -121,6 +129,8 @@ private:
     bool                         m_renameJustStarted  = false;
 
     void         HandleKeyboardShortcuts();
+    void         ToggleSelectionFromClick(const std::string& path);
+    void         RangeSelectToPath(const pxr::SdfPath& path);
     SdfPath      FindUniqueChildPath(const SdfPath& parent, const std::string& baseName);
 };