address review: stop TEXT rendering past the cell limit and count UTF-16 units

A text section repeats its input once per @ placeholder, and TEXT handed
back whatever that built, so nested TEXT calls could multiply a long
text until memory ran out. Results past the cell text limit are now
#VALUE!.

crates/numfmt/src/lib.rs

@@ -332,6 +332,14 @@ impl NumberFormat {
             None => Formatted { text: t.to_string(), color: None, fill: None, numeric: false },
         }
     }
+
+    /// [`Self::format_text`]'s text, or `None` once it passes `max_units` UTF-16 code units.
+    fn format_text_within(&self, t: &str, max_units: usize) -> Option<String> {
+        match self.inner.text_section.and_then(|i| self.inner.sections.get(i)) {
+            Some(sec) => render::render_text_within(sec, t, max_units).map(|out| out.text),
+            None => (t.encode_utf16().count() <= max_units).then(|| t.to_string()),
+        }
+    }
 }
 
 /// Formats a cell value for display.
@@ -425,6 +433,10 @@ impl NumberFormat {
 /// The `TEXT()` worksheet function: formats a value with a format code. Numeric text is
 /// converted to a number first; other text and booleans (as `TRUE`/`FALSE`) go through the text
 /// section, or are returned as is.
+/// Longest text a cell holds, in UTF-16 code units (Excel's 32,767 characters, which XLSB also
+/// stores that way), and so the longest `TEXT` result.
+const MAX_CELL_TEXT_CHARS: usize = 32_767;
+
 pub fn text_function(v: &Value, code: &str, sys: DateSystem) -> Result<String, CellError> {
     if code.is_empty() {
         return Ok(String::new());
@@ -440,7 +452,9 @@ pub fn text_function(v: &Value, code: &str, sys: DateSystem) -> Result<String, C
         Value::Bool(b) => return Ok(fmt.format_text(if b { "TRUE" } else { "FALSE" }).text),
         Value::Text(t) => match gridcraft_core::parse::parse_number_text(&t) {
             Some(n) => n,
-            None => return Ok(fmt.format_text(&t).text),
+            // Every `@` repeats the text: a result past a cell's limit is `#VALUE!`, so nested
+            // TEXT calls can't multiply it further.
+            None => return fmt.format_text_within(&t, MAX_CELL_TEXT_CHARS).ok_or(CellError::Value),
         },
         Value::Array(_) => return Err(CellError::Value),
     };

crates/numfmt/src/render.rs

@@ -45,6 +45,35 @@ pub(crate) fn render_text(sec: &Section, text: &str) -> Out {
     out
 }
 
+/// [`render_text`], but `None` once the output passes `max_units` UTF-16 code units (a cell's
+/// limit), checked before each piece is appended, so an oversized result is never built.
+pub(crate) fn render_text_within(sec: &Section, text: &str, max_units: usize) -> Option<Out> {
+    let text_units = text.encode_utf16().count();
+    let mut out = Out::default();
+    let mut units = 0usize;
+    for t in &sec.toks {
+        match t {
+            Tok::At => {
+                units = units.saturating_add(text_units);
+                if units > max_units {
+                    return None;
+                }
+                out.push(text);
+            }
+            Tok::Fill(c) => out.fill(*c),
+            other => {
+                let literal = other.literal_text();
+                units = units.saturating_add(literal.encode_utf16().count());
+                if units > max_units {
+                    return None;
+                }
+                out.push(&literal);
+            }
+        }
+    }
+    Some(out)
+}
+
 /// Renders a number through a General section (literals around the General keyword).
 pub(crate) fn render_general(sec: &Section, v: f64, width: usize) -> Out {
     let mut out = Out::default();

crates/numfmt/src/tests.rs

@@ -396,6 +396,13 @@ fn text_fn() {
     assert_eq!(text_function(&Value::Number(1234.5), "$#,##0.00", S), Ok("$1,234.50".into()));
     assert_eq!(text_function(&Value::from("5"), "0.00", S), Ok("5.00".into()));
     assert_eq!(text_function(&Value::from("abc"), "0.00", S), Ok("abc".into()));
+    // Each `@` repeats the text: up to a cell's 32,767 characters, then #VALUE!.
+    let at = "@".repeat(255);
+    assert_eq!(text_function(&Value::from("x".repeat(128).as_str()), &at, S).map(|s| s.len()), Ok(128 * 255));
+    assert_eq!(text_function(&Value::from("x".repeat(32_000).as_str()), &at, S), Err(CellError::Value));
+    // The limit counts UTF-16 units: 16,384 emoji are 32,768 of them, one too many.
+    assert_eq!(text_function(&Value::from("\u{1F600}".repeat(16_383).as_str()), "@", S).map(|s| s.chars().count()), Ok(16_383));
+    assert_eq!(text_function(&Value::from("\u{1F600}".repeat(16_384).as_str()), "@", S), Err(CellError::Value));
     assert_eq!(text_function(&Value::Error(CellError::NA), "0", S), Err(CellError::NA));
     assert_eq!(text_function(&Value::Number(46302.0), "yyyy", S), Ok("2026".into()));
     assert_eq!(text_function(&Value::Number(0.5), "General", S), Ok("0.5".into()));