gfx: fix off-screen default position of the Text node

An unconfigured Text process rendered nothing: the UBO default position was
{0.5,0.5} (TextNode.hpp), a clip-space offset added to the text quad in the
vertex shader, which shoved the default text ~half a screen up+right — off the
top edge — until Position was manually set. Default to {0,0} (screen centre).

Found + verified by the text-render harness (untouched Text now renders
1508 visible px; the harness's default-visibility check is a hard assertion).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014rZgzE8JjWvHDtaVUhxpLE

Author: jcelerier

src/plugins/score-plugin-gfx/Gfx/Graph/TextNode.hpp

@@ -26,7 +26,11 @@ struct TextNode : NodeModel
   {
     float opacity{1.};
     float padding_std140_0{};
-    float position[2]{0.5, 0.5};
+    // Clip-space offset (added to the text quad in the vertex shader). {0,0} is
+    // screen centre; the previous {0.5,0.5} default shifted an unconfigured Text
+    // process ~half a screen up+right — off the top edge — so default text was
+    // invisible until Position was touched. (video/text render validation)
+    float position[2]{0.0, 0.0};
     float scale[2]{1., 1.};
   } ubo;
 #pragma pack(pop)