Scene hierarchy: add Inherit, Specialize, and VariantSet composition arc menus

Prim right-click context menu now exposes all five USD composition arc types:

- Inherit▶  Add... (modal for SdfPath input) | Remove▶ (lists via GetAllDirectInherits()) | Clear All
- Specialize▶  Add... | Remove▶ (lists via UsdPrimCompositionQuery + ArcTypeFilter::Specialize) | Clear All
- VariantSet▶  Add... | per-set sub-menu: Add Variant... + selectable variant list (SetVariantSelection)

All text-input operations share a single RenderArcModal() driven by ArcModalMode enum.
VariantSet/Variant names go through SanitizeUsdName(); Inherit/Specialize paths are passed
as-is to SdfPath. OK button is disabled until input is non-empty; Enter key also confirms.

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

src/ui/SceneHierarchyPanel.h

@@ -66,12 +66,15 @@ public:
     using StageMetadataChangedCallback = std::function<void()>;
     void SetOnStageMetadataChanged(StageMetadataChangedCallback callback) { m_onStageMetadataChanged = callback; }
     
+    enum class ArcModalMode { None, Inherit, Specialize, VariantSet, Variant };
+
 private:
     void RenderPrimNode(const UsdPrim& prim);
     const char* GetPrimTypeIcon(const UsdPrim& prim) const;
     Icon        GetPrimTypeIconEnum(const UsdPrim& prim) const;
     void RenderContextMenu(const UsdPrim& prim);
     void RenderRemovePrimModal();
+    void RenderArcModal();
     void ProcessPendingReplaceRef();
     
     PropertyManager*             m_propertyManager;
@@ -104,6 +107,13 @@ private:
     bool                         m_doReplaceRefPick  = false;
     SdfPath                      m_pendingReplaceRefPrim;
     pxr::SdfReference            m_pendingReplaceRef;
+
+    /// Arc modal state (Inherit / Specialize / VariantSet / Variant).
+    ArcModalMode                 m_arcModalMode       = ArcModalMode::None;
+    SdfPath                      m_arcModalTargetPrim;
+    std::string                  m_arcModalVarSetName;
+    char                         m_arcModalBuf[512]   = {};
+    bool                         m_openArcModal       = false;
 };
 
 } // namespace UsdLayerManager