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/generator.h

@@ -0,0 +1,18 @@
+#pragma once
+#include "core.h"
+#include <QString>
+#include <QSet>
+
+namespace rcx {
+
+// Generate C++ struct definitions for a single root struct and all
+// nested/referenced types reachable from it.
+QString renderCpp(const NodeTree& tree, uint64_t rootStructId);
+
+// Generate C++ struct definitions for every root-level struct (full SDK).
+QString renderCppAll(const NodeTree& tree);
+
+// Null generator placeholder (returns empty string).
+QString renderNull(const NodeTree& tree, uint64_t rootStructId);
+
+} // namespace rcx