G4: two windows on one workbook without copying it; Arrange All

- The workbook's active sheet is an atomic set in place (Workbook::active_sheet /
  set_active_sheet), so a window swap or sheet switch never clones the
  Arc<Workbook> or marks it dirty. Selections already live in DocState and the
  window views. Save records the sheet of the most recently active window.
- A dialog taken out of its window while it runs goes back to that window when
  a command run from it switched windows (dialogs::show, ui.dialog.confirm).
- view.arrangeAll {arrange?: tiled|horizontal|vertical|cascade, activeWorkbook?}
  returns the visible windows, active first; the desktop moves and sizes the
  native windows over the main window's monitor. New Window and Arrange All
  buttons on the View ribbon; Arrange Windows dialog.
- Docs: ui-parity, hardware-parity, gaps #20, target-app-parity, ROADMAP.

crates/engine/src/sample.rs

@@ -37,7 +37,7 @@ pub fn build(name: &str) -> Option<Workbook> {
         return None;
     }
     let mut wb = (*s.doc().ok()?.wb).clone();
-    wb.active_sheet = 0;
+    wb.set_active_sheet(0);
     if let Some(sh) = wb.sheet_mut(0) {
         sh.view_active = gridcraft_core::CellRef::default();
     }