Hypershade-style pin display modes for material graph nodes

Each node gets an All/Conn toggle: connected-only mode hides
unconnected pins behind collapsed connector nubs on the title row;
dragging a link onto (or from) a nub opens a menu to pick which hidden
pin to wire, and the pin appears once connected. The toggle is drawn
and hit-tested geometrically, and its tooltip is deferred to after
NE::End() so it lands at the cursor. A new Preferences > Material
checkbox opts into persisting the per-node mode as USD customData
(uiShowAllPins); otherwise it stays session-only.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/ui/Application.h

@@ -30,6 +30,11 @@ struct AppPreferences {
     std::string ocioLook;
     float materialBrowserWidth = 220.0f; ///< Material Editor left-column width
     float materialPreviewWidth = 320.0f; ///< Material Editor right-column width
+    /// When true, per-node pin display mode (Hypershade-style "show all" vs
+    /// "show connected only") and revealed hidden pins are authored into USD
+    /// customData so they survive reopening the material; when false, that
+    /// state is session-only.
+    bool keepGraphNodeViewSettingsInUsd = false;
 };
 
 class Application {