tidy(cli): pluralization-aware zero-trade note wording

Milestone-fieldtest friction: a single-OOS-window walk-forward printed
"all 1 walk-forward windows recorded zero trades" (captured RED
evidence: fieldtests/milestone-stderr-honesty/captured/
example2_singular.err). The message text moves into a pure
zero-trade_note_text builder — singular "the single walk-forward
window recorded zero trades", plural unchanged so the existing e2e
phrase pins hold — with a unit pin on both forms. refs #278

crates/aura-cli/src/diag.rs

@@ -19,6 +19,19 @@ macro_rules! warning {
 
 pub(crate) use {note, warning};
 
+/// The zero-trade note's message text (#313), pluralization-aware — the
+/// milestone fieldtest tripped over "all 1 walk-forward windows"
+/// (captured: `fieldtests/milestone-stderr-honesty/captured/
+/// example2_singular.err`). Pure over the count so the wording is
+/// unit-pinned.
+fn zero_trade_note_text(n_windows: usize) -> String {
+    if n_windows == 1 {
+        "the single walk-forward window recorded zero trades".to_string()
+    } else {
+        format!("all {n_windows} walk-forward windows recorded zero trades")
+    }
+}
+
 /// The #313 zero-trade note, shared by both walk-forward paths (the
 /// synthetic-blueprint path in `main.rs` and the `--real` sugar path in
 /// `verb_sugar.rs`) so the wording AND the condition live in exactly one
@@ -27,6 +40,26 @@ pub(crate) use {note, warning};
 pub(crate) fn note_zero_trade_windows(mut window_trades: impl ExactSizeIterator<Item = u64>) {
     let n_windows = window_trades.len();
     if n_windows > 0 && window_trades.all(|n| n == 0) {
-        note!("all {n_windows} walk-forward windows recorded zero trades");
+        note!("{}", zero_trade_note_text(n_windows));
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::zero_trade_note_text;
+
+    #[test]
+    /// #278 fieldtest friction: the single-window form must read as
+    /// grammatical English while the plural form keeps the exact phrase
+    /// the e2e pins grep for.
+    fn zero_trade_note_pluralizes() {
+        assert_eq!(
+            zero_trade_note_text(1),
+            "the single walk-forward window recorded zero trades"
+        );
+        assert_eq!(
+            zero_trade_note_text(3),
+            "all 3 walk-forward windows recorded zero trades"
+        );
     }
 }