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/formulas.rs

@@ -102,7 +102,7 @@ fn auto_sum(s: &mut Session, p: &Json) -> Result<Json> {
                 (CellRef::new(out_row, c), format!("={func}({})", RangeRef::new(CellRef::new(sel.start.row, c), CellRef::new(data_end_row, c)).a1()))
             })
             .collect();
-        let sheet = d.wb.active_sheet;
+        let sheet = d.wb.active_sheet();
         edit(s, |cx| {
             for (c, f) in &writes {
                 let cell = super::edit::input_to_cell(f, sheet, *c, &mut cx.wb)?.unwrap_or_default();
@@ -400,7 +400,7 @@ fn name_manager(s: &mut Session, _: &Json) -> Result<Json> {
         .iter()
         .filter(|n| !n.hidden)
         .map(|n| {
-            let sheet = n.scope.unwrap_or(d.wb.active_sheet);
+            let sheet = n.scope.unwrap_or(d.wb.active_sheet());
             // parse_name, not evaluate's parse: a name's top-level commas are unions.
             let v = gridcraft_formula::parse_name(&n.formula)
                 .map_or(Value::Error(CellError::Name), |e| gridcraft_calc::recalc::evaluate_expr(&d.wb, sheet, CellRef::default(), &e));
@@ -477,7 +477,7 @@ fn create_from_selection(s: &mut Session, p: &Json) -> Result<Json> {
 fn trace_precedents(s: &mut Session, p: &Json) -> Result<Json> {
     let d = s.doc()?;
     let at = cell_param(s, p, "cell")?.unwrap_or(d.selection.active);
-    let areas = d.calc.graph.precedents_of((d.wb.active_sheet, at));
+    let areas = d.calc.graph.precedents_of((d.wb.active_sheet(), at));
     let v: Vec<Json> = areas.iter().map(|a| json!({"sheet": d.wb.sheet(a.sheet).map(|s| s.name.clone()), "range": a.range.a1()})).collect();
     Ok(Json::Array(v))
 }
@@ -486,7 +486,7 @@ fn trace_dependents(s: &mut Session, p: &Json) -> Result<Json> {
     let d = s.doc()?;
     let at = cell_param(s, p, "cell")?.unwrap_or(d.selection.active);
     let mut deps = Vec::new();
-    d.calc.graph.dependents(d.wb.active_sheet, at, &mut deps);
+    d.calc.graph.dependents(d.wb.active_sheet(), at, &mut deps);
     deps.sort();
     deps.dedup();
     let v: Vec<Json> = deps.iter().map(|(si, c)| json!({"sheet": d.wb.sheet(*si).map(|s| s.name.clone()), "cell": c.a1()})).collect();
@@ -499,7 +499,7 @@ fn select_trace_cells(s: &mut Session, p: &Json, precedents: bool) -> Result<Jso
     use std::collections::BTreeSet;
     let d = s.doc()?;
     let at = cell_param(s, p, "cell")?.unwrap_or(d.selection.active);
-    let sheet = d.wb.active_sheet;
+    let sheet = d.wb.active_sheet();
     let all = bool_param(p, "all").unwrap_or(false);
     // (sheet, a1) pairs to select; direct level keeps whole precedent ranges, transitive cells.
     let mut found: BTreeSet<(usize, String)> = BTreeSet::new();
@@ -570,7 +570,7 @@ fn select_dependents(s: &mut Session, p: &Json) -> Result<Json> {
 }
 
 fn show_formulas(s: &mut Session, p: &Json) -> Result<Json> {
-    let sheet = s.doc()?.wb.active_sheet;
+    let sheet = s.doc()?.wb.active_sheet();
     let cur = s.doc()?.wb.sheet(sheet).is_some_and(|sh| sh.show_formulas);
     let on = bool_param(p, "on").unwrap_or(!cur);
     edit(s, |cx| {
@@ -636,7 +636,7 @@ fn error_checking(s: &mut Session, _: &Json) -> Result<Json> {
 fn evaluate_formula(s: &mut Session, p: &Json) -> Result<Json> {
     let d = s.doc()?;
     let at = cell_param(s, p, "cell")?.unwrap_or(d.selection.active);
-    let sheet = d.wb.active_sheet;
+    let sheet = d.wb.active_sheet();
     let text = match str_param(p, "formula") {
         Some(f) => f.trim_start_matches('=').to_string(),
         None => {
@@ -675,7 +675,7 @@ fn evaluate(s: &mut Session, p: &Json) -> Result<Json> {
     let at = cell_param(s, p, "cell")?.unwrap_or(d.selection.active);
     let expr = crate::cmd::edit::parse_typed_formula(f.strip_prefix('=').unwrap_or(f))?;
     crate::cmd::edit::check_arg_counts(&expr)?;
-    let v = gridcraft_calc::recalc::evaluate_expr(&d.wb, d.wb.active_sheet, at, &expr);
+    let v = gridcraft_calc::recalc::evaluate_expr(&d.wb, d.wb.active_sheet(), at, &expr);
     Ok(crate::cmd::inspect::value_json(&v))
 }