fix(formats): build at most MAX_COLS cells per Markdown table row

crates/formats/src/markdown/blocks.rs

@@ -474,14 +474,17 @@ fn parse_blocks(lines: &[String], depth: usize, level: u8, refs: &Refs) -> Vec<F
                     let p = Para { align: a, inlines: parse_inlines(s, &Fmt::default(), depth, refs), ..Default::default() };
                     Cell { blocks: vec![FBlock::Para(p)], header, ..Default::default() }
                 };
-                table.rows.push(head.iter().zip(&aligns).map(|(s, a)| cell(s, true, *a)).collect());
+                // A document row keeps MAX_COLS cells: only those are built, so a wide header
+                // doesn't pad every sparse row below it out to its width.
+                let cols = aligns.len().min(wordcraft_doc::table::MAX_COLS);
+                table.rows.push(head.iter().zip(&aligns).take(cols).map(|(s, a)| cell(s, true, *a)).collect());
                 i += 2;
                 while let Some(l) = lines.get(i) {
                     if is_blank(l) || !l.contains('|') || interrupts(l) {
                         break;
                     }
                     let mut cells = split_row(l);
-                    cells.resize(aligns.len(), String::new());
+                    cells.resize(cols, String::new());
                     table.rows.push(cells.iter().zip(&aligns).map(|(s, a)| cell(s, false, *a)).collect());
                     i += 1;
                 }

crates/formats/src/markdown/tests.rs

@@ -714,3 +714,17 @@ fn exported_link_destinations_read_back_unchanged() {
     assert_eq!(md("https://example.com/a b"), "[x](<https://example.com/a b>)");
     assert_eq!(md("https://e.com/x\ny"), "[x](https://e.com/xy)");
 }
+
+#[test]
+fn a_wide_table_builds_only_the_cells_a_row_keeps() {
+    // A 5,000-column header padded every one-cell row below it to 5,000 cells.
+    let n = 5000;
+    let md = format!("{}|\n{}|\n{}", "|h".repeat(n), "|-".repeat(n), "|x|\n".repeat(200));
+    let flow = parse(&md);
+    let FBlock::Table(t) = &flow.blocks[0] else { panic!("expected table") };
+    assert_eq!(t.rows.len(), 201);
+    assert!(t.rows.iter().all(|r| r.len() == wordcraft_doc::table::MAX_COLS));
+    // A narrow table is unchanged.
+    let FBlock::Table(t) = &parse("| a | b |\n| - | - |\n| x |\n").blocks[0] else { panic!("expected table") };
+    assert_eq!(t.rows[1].len(), 2);
+}

crates/formats/src/sheet/xlsx.rs

@@ -1,5 +1,3 @@
-use std::borrow::Cow;
-
 use super::date::{DateKind, builtin_date, custom_date, iso_datetime, serial_date};
 use super::{
     BTreeMap, Ev, Grid, HashMap, MAX_CELLS, MAX_COLS, MAX_ROWS, MAX_SHARED, MAX_SHEETS, MAX_XFS, Package, TextBudget, attr, push_capped, walk,
@@ -263,7 +261,7 @@ pub(super) fn xlsx_rows(pkg: &mut Package<'_>, path: &str) -> Result<Vec<Vec<Str
                     row.resize(c.col + 1, String::new());
                 }
                 if let Some(slot) = row.get_mut(c.col) {
-                    *slot = text.into_owned();
+                    *slot = text;
                 }
             }
             _ => {}
@@ -276,15 +274,12 @@ pub(super) fn xlsx_rows(pkg: &mut Package<'_>, path: &str) -> Result<Vec<Vec<Str
     Ok(grid.rows)
 }
 
-/// A cell's text: a shared string borrowed, so a cell the sheet then refuses (past the text
-/// budget, the columns or the rows) costs no copy of it.
-fn xlsx_value<'a>(c: &XCell, shared: &'a [String], formats: &[Option<DateKind>], date1904: bool) -> Cow<'a, str> {
+fn xlsx_value(c: &XCell, shared: &[String], formats: &[Option<DateKind>], date1904: bool) -> String {
     let v = c.value.trim();
     let kind = c.kind.trim();
     if kind.eq_ignore_ascii_case("s") {
-        return Cow::Borrowed(v.parse::<usize>().ok().and_then(|i| shared.get(i)).map_or("", String::as_str));
-    }
-    Cow::Owned(if kind.eq_ignore_ascii_case("inlineStr") {
+        v.parse::<usize>().ok().and_then(|i| shared.get(i)).cloned().unwrap_or_default()
+    } else if kind.eq_ignore_ascii_case("inlineStr") {
         if c.inline.is_empty() { c.value.clone() } else { c.inline.clone() }
     } else if kind.eq_ignore_ascii_case("str") || kind.eq_ignore_ascii_case("e") {
         c.value.clone()
@@ -295,12 +290,12 @@ fn xlsx_value<'a>(c: &XCell, shared: &'a [String], formats: &[Option<DateKind>],
     } else if v.is_empty() {
         c.inline.clone()
     } else {
-        let Ok(x) = v.parse::<f64>() else { return Cow::Owned(v.to_string()) };
+        let Ok(x) = v.parse::<f64>() else { return v.to_string() };
         match formats.get(c.style).copied().flatten() {
             Some(kind) => serial_date(x, date1904, kind).unwrap_or_else(|| number(x)),
             None => number(x),
         }
-    })
+    }
 }
 
 /// A number as a spreadsheet shows it in General format: up to 15 significant digits, no