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/format.rs

@@ -1011,7 +1011,7 @@ fn line_targets(s: &Session, p: &Json, rows: bool) -> Result<(usize, Vec<(u32, u
         return Ok((sheet, lines));
     }
     let d = s.doc()?;
-    if sheet != d.wb.active_sheet {
+    if sheet != d.wb.active_sheet() {
         return Err(bad(key, format!("missing `{key}`: the selection is on the active sheet")));
     }
     Ok((sheet, d.selection.ranges.iter().map(|r| if rows { (r.start.row, r.end.row) } else { (r.start.col, r.end.col) }).collect()))