numfmt: replace double-reverse with forward pass in apply_grouping

Build the grouped string in a single forward pass using chunk iteration
instead of reversing, inserting separators, then reversing again. This
avoids an extra String allocation and collection.

Author: sylvestre

src/uu/numfmt/src/format.rs

@@ -206,20 +206,26 @@ fn apply_grouping(s: &str) -> String {
         return s.to_string();
     }
 
-    let mut grouped_rev = String::with_capacity(s.len() + (integer.len() / 3));
-    for (idx, ch) in integer.chars().rev().enumerate() {
-        if idx > 0 && idx % 3 == 0 {
-            grouped_rev.push_str(grouping_separator);
-        }
-        grouped_rev.push(ch);
+    let sep_len = grouping_separator.len();
+    let num_seps = (integer.len() - 1) / 3;
+    let mut grouped = String::with_capacity(sign.len() + integer.len() + num_seps * sep_len + if fraction.is_empty() { 0 } else { 1 + fraction.len() });
+    grouped.push_str(sign);
+
+    let first_group = integer.len() % 3;
+    let first_group = if first_group == 0 { 3 } else { first_group };
+    grouped.push_str(&integer[..first_group]);
+    for chunk in integer[first_group..].as_bytes().chunks(3) {
+        grouped.push_str(grouping_separator);
+        // SAFETY: integer is known to be valid UTF-8 ASCII digits
+        grouped.push_str(std::str::from_utf8(chunk).unwrap());
     }
-    let grouped_integer: String = grouped_rev.chars().rev().collect();
 
-    if fraction.is_empty() {
-        format!("{sign}{grouped_integer}")
-    } else {
-        format!("{sign}{grouped_integer}.{fraction}")
+    if !fraction.is_empty() {
+        grouped.push('.');
+        grouped.push_str(fraction);
     }
+
+    grouped
 }
 
 fn next_field_index(s: &str) -> usize {