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/data.rs

@@ -158,7 +158,7 @@ fn data_target(s: &Session, p: &Json, cmd: &str) -> Result<(usize, RangeRef)> {
     {
         return Err(bad(cmd, format!("`{t}` names a sheet other than `sheet`")));
     }
-    if sheet != d.wb.active_sheet && p.get("range").is_none() {
+    if sheet != d.wb.active_sheet() && p.get("range").is_none() {
         return Err(bad(cmd, "missing `range`: the selection is on the active sheet"));
     }
     Ok((sheet, data_range(s, p)?))
@@ -210,7 +210,7 @@ fn sort_target(s: &Session, p: &Json) -> Result<(usize, RangeRef)> {
     {
         return Err(bad("data.sort", format!("`{t}` names a sheet other than `sheet`")));
     }
-    if sheet != d.wb.active_sheet && p.get("range").is_none() {
+    if sheet != d.wb.active_sheet() && p.get("range").is_none() {
         return Err(bad("data.sort", "missing `range`: the selection is on the active sheet"));
     }
     Ok((sheet, data_range(s, p)?))
@@ -502,7 +502,7 @@ fn toggle_filter(s: &mut Session, p: &Json) -> Result<Json> {
     let (sheet, r) = data_target(s, p, "data.filter")?;
     super::refuse_unless_allowed(s, sheet, "Filtering", |p| p.autofilter)?;
     // The active cell belongs to the active sheet: on another sheet the table is the one at `range`.
-    let at = if sheet == s.doc()?.wb.active_sheet { s.doc()?.selection.active } else { r.start };
+    let at = if sheet == s.doc()?.wb.active_sheet() { s.doc()?.selection.active } else { r.start };
     let has = s.doc()?.wb.sheet(sheet).is_some_and(|sh| sh.autofilter.is_some());
     let in_table = s.doc()?.wb.sheet(sheet).and_then(|sh| sh.table_at(at).map(|t| t.name.clone()));
     if let Some(name) = in_table {
@@ -554,7 +554,7 @@ fn install_filter(s: &mut Session, p: &Json) -> Result<Json> {
 }
 
 fn filter_by(s: &mut Session, p: &Json) -> Result<Json> {
-    let sheet = s.doc()?.wb.active_sheet;
+    let sheet = s.doc()?.wb.active_sheet();
     super::refuse_unless_allowed(s, sheet, "Filtering", |p| p.autofilter)?;
     let Some(sh) = s.doc()?.wb.sheet(sheet) else { return Err(EngineError::NoDocument) };
     let af = match &sh.autofilter {
@@ -741,7 +741,7 @@ pub(crate) fn apply_filter(wb: &mut Workbook, sheet: usize) {
 }
 
 fn clear_filter(s: &mut Session, _: &Json) -> Result<Json> {
-    let sheet = s.doc()?.wb.active_sheet;
+    let sheet = s.doc()?.wb.active_sheet();
     edit(s, |cx| {
         let sh = cx.sheet_mut(sheet)?;
         if let Some(a) = sh.autofilter.as_mut() {
@@ -754,7 +754,7 @@ fn clear_filter(s: &mut Session, _: &Json) -> Result<Json> {
 }
 
 fn reapply(s: &mut Session, _: &Json) -> Result<Json> {
-    let sheet = s.doc()?.wb.active_sheet;
+    let sheet = s.doc()?.wb.active_sheet();
     edit(s, |cx| {
         apply_filter(&mut cx.wb, sheet);
         cx.structural = true;
@@ -1258,7 +1258,7 @@ fn validate_cmd(s: &mut Session, p: &Json) -> Result<Json> {
     let d = s.doc()?;
     let at = cell_param(s, p, "cell")?.unwrap_or(d.selection.active);
     let input = str_param(p, "input").unwrap_or("");
-    match check_validation(&d.wb, d.wb.active_sheet, at, input) {
+    match check_validation(&d.wb, d.wb.active_sheet(), at, input) {
         None => Ok(json!({"ok": true})),
         Some((dv, msg)) => Ok(json!({"ok": false, "message": msg, "title": dv.error_title, "style": format!("{:?}", dv.error_style)})),
     }
@@ -1266,7 +1266,7 @@ fn validate_cmd(s: &mut Session, p: &Json) -> Result<Json> {
 
 fn circle_invalid(s: &mut Session, _: &Json) -> Result<Json> {
     let d = s.doc()?;
-    let sheet = d.wb.active_sheet;
+    let sheet = d.wb.active_sheet();
     let Some(sh) = d.wb.sheet(sheet) else { return Err(EngineError::NoDocument) };
     let mut bad_cells = Vec::new();
     for dv in &sh.validations {
@@ -1305,7 +1305,7 @@ fn outline_lines(s: &Session, p: &Json) -> Result<(bool, u32, u32)> {
 fn group(s: &mut Session, p: &Json, on: bool) -> Result<Json> {
     let si = target_sheet(s, p)?;
     // The selection belongs to the active sheet, so another sheet needs its lines.
-    if si != s.doc()?.wb.active_sheet && p.get("rows").is_none() && p.get("cols").is_none() {
+    if si != s.doc()?.wb.active_sheet() && p.get("rows").is_none() && p.get("cols").is_none() {
         return Err(bad(if on { "data.group" } else { "data.ungroup" }, "missing `rows` or `cols`: the selection is on the active sheet"));
     }
     let (rows, a, b) = outline_lines(s, p)?;