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/cmd/mod.rs

@@ -271,7 +271,7 @@ pub(crate) fn target_sheet(s: &Session, p: &Json) -> Result<usize> {
     {
         return d.wb.sheet_index(&sh).ok_or_else(|| bad("range", format!("no sheet named `{sh}`")));
     }
-    Ok(d.wb.active_sheet)
+    Ok(d.wb.active_sheet())
 }
 
 pub(crate) use gridcraft_core::split_sheet;
@@ -280,7 +280,7 @@ pub(crate) use gridcraft_core::split_sheet;
 /// a cell reference, or that names a sheet other than the active one, is an error rather than a
 /// silent fallback to some other cell.
 pub(crate) fn cell_param(s: &Session, p: &Json, key: &str) -> Result<Option<CellRef>> {
-    cell_param_on(s, p, key, s.doc()?.wb.active_sheet)
+    cell_param_on(s, p, key, s.doc()?.wb.active_sheet())
 }
 
 /// Like [`cell_param`], for a command that operates on `sheet` (a `Sheet!` prefix must name it).
@@ -562,7 +562,7 @@ pub fn auto_row_height(wb: &mut Workbook, si: usize, row: u32) {
 
 impl Ctx<'_> {
     pub fn sheet_index(&self) -> usize {
-        self.wb.active_sheet
+        self.wb.active_sheet()
     }
     pub fn sheet_mut(&mut self, i: usize) -> Result<&mut gridcraft_model::Sheet> {
         self.wb.sheet_mut(i).ok_or_else(|| EngineError::Other("no such sheet".into()))