Print numbers near f64::MAX in a form that parses back

Found while testing crates/formula/src/printer.rs:134

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/formula/src/printer.rs

@@ -132,7 +132,16 @@ fn write_struct_ref(s: &StructRef, at_shorthand: bool) -> String {
 }
 
 fn number(n: f64) -> String {
-    number_to_text(n)
+    let text = number_to_text(n);
+    // 15 significant digits can round past f64::MAX, which no longer parses.
+    if text.parse::<f64>().is_ok_and(f64::is_finite) {
+        return text;
+    }
+    let exact = format!("{:E}", n);
+    match exact.split_once('E') {
+        Some((m, e)) if !e.starts_with('-') => format!("{m}E+{e}"),
+        _ => exact,
+    }
 }
 
 /// Prints an expression in A1 notation.

crates/formula/tests/printer_roundtrip.rs

@@ -0,0 +1,15 @@
+use gridcraft_formula::{Expr, parse, print};
+
+#[test]
+fn number_near_f64_max_round_trips() {
+    for src in ["=1.7976931348623157E308", "=-1.7976931348623157E308"] {
+        let e = parse(src).unwrap();
+        let printed = print(&e);
+        let back = parse(&printed).unwrap_or_else(|err| panic!("{printed}: {err:?}"));
+        assert_eq!(print(&back), printed);
+        match (&e, &back) {
+            (Expr::Number(a), Expr::Number(b)) => assert_eq!(a, b),
+            _ => assert_eq!(e, back),
+        }
+    }
+}