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/view.rs

@@ -139,7 +139,7 @@ fn freeze_panes(s: &mut Session, p: &Json) -> Result<Json> {
     }
     let at = match cell_param_on(s, p, "cell", si)? {
         Some(c) => c,
-        None if si == d.wb.active_sheet => d.selection.active,
+        None if si == d.wb.active_sheet() => d.selection.active,
         // The selection belongs to the active sheet.
         None => return Err(bad("view.freezePanes", "missing `cell`: the selection is on the active sheet")),
     };
@@ -190,7 +190,7 @@ fn zoom(s: &mut Session, p: &Json) -> Result<Json> {
     let d = s.doc_mut()?;
     let dirty = d.is_dirty();
     let wb = std::sync::Arc::make_mut(&mut d.wb);
-    let si = wb.active_sheet;
+    let si = wb.active_sheet();
     if let Some(sh) = wb.sheet_mut(si) {
         sh.zoom = z;
     }