Add class keyword picker, alignas alignment picker with member realignment

- Class keyword (struct/class/union/enum) persists in JSON, drives generator output
- CommandRow2 shows alignas(N) pill computed from struct member alignment
- Clicking alignas opens picker (1, 4, 8, 16) to realign all members
- Going up: members repositioned to N-byte boundaries, padding fills gaps
- Going down: excess padding removed, members pack tighter
- Add cmd::ChangeOffset for undoable offset repositioning
- All 11 tests pass

Co-Authored-By: combuter <combuter@users.noreply.github.com>

src/controller.h

@@ -128,6 +128,7 @@ private:
     void connectEditor(RcxEditor* editor);
     void handleMarginClick(RcxEditor* editor, int margin, int line, Qt::KeyboardModifiers mods);
     void updateCommandRow();
+    void performRealignment(uint64_t structId, int targetAlign);
     void attachToProcess(uint32_t pid, const QString& processName);
     void switchToSavedSource(int idx);
     void pushSavedSourcesToEditors();