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/ViewportPanel.h

@@ -48,6 +48,8 @@ public:
     // ── Selection (called by SceneHierarchyPanel) ────────────────────────────
     /// Set a single selected prim (clears any multi-selection).
     void SetSelectedPrimPath(const std::string& path);
+    /// Set multiple selected prims (from hierarchy Ctrl+click multi-select).
+    void SetSelectedPrimPaths(const std::vector<std::string>& paths);
 
     // ── Main render (called from Application::RenderUI) ──────────────────────
     void Render(bool* p_open = nullptr);
@@ -99,7 +101,7 @@ private:
     bool IsMouseOverDivider(ImVec2 origin, ImVec2 total) const;
 
     /// Width (px) of the reserved left toolbar strip.
-    static constexpr float kToolbarW = 52.0f;
+    static constexpr float kToolbarW = 40.0f;
 
     // ── Selection management ─────────────────────────────────────────────────
     /// Push the current shared selection into every tile and the manipulator.