Add C++ code generator, rendered view, fix Hex node display

- Add generator.h/cpp: C++ struct emission with padding gaps, tail
  padding, pragma pack, static_assert, nested structs, pointers
- Add rendered view (QStackedWidget per tab, QsciScintilla + C++ lexer)
- View menu: C/C++ switches to rendered view, Reclass View switches back
- File > Export C++ Header writes full SDK to .h file
- Fix Hex8-Hex64 display: show name + value columns like normal types
  instead of hiding name behind ASCII preview (only Padding keeps that)
- Update column span detection, compose width calc, controller edit
  dispatch, and editor dimming to match new Hex layout
- Sample data: Entity struct with health, armor, speed, flags fields
- Add test_generator (21 tests) and test_validation

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

src/controller.cpp

@@ -172,8 +172,8 @@ void RcxController::connectEditor(RcxEditor* editor) {
         case EditTarget::Name: {
             if (text.isEmpty()) break;
             const Node& node = m_doc->tree.nodes[nodeIdx];
-            // ASCII edit on Hex/Padding nodes
-            if (isHexPreview(node.kind)) {
+            // ASCII edit on Padding nodes
+            if (node.kind == NodeKind::Padding) {
                 setNodeValue(nodeIdx, subLine, text, /*isAscii=*/true);
             } else {
                 renameNode(nodeIdx, text);