fix(proto): congestion: avoid double-reducing CUBIC fast convergence

Quinn mistakenly applied the beta multiplier to the newly adjusted `W_max` instead of the current window.
This double-reduced the window to 59.5% instead of the intended 70%.
This fixes the math by snapshotting the original window for the multiplicative decrease.

noq-proto/src/congestion/cubic.rs

@@ -199,18 +199,19 @@ impl Controller for Cubic {
         }
 
         self.state.recovery_start_time = Some(now);
+        let window = self.state.window as f64;
 
-        // Fast convergence
-        if (self.state.window as f64) < self.state.w_max {
-            self.state.w_max = self.state.window as f64 * (1.0 + BETA_CUBIC) / 2.0;
+        // Fast convergence lowers W_max first; the 0.7 loss reduction still
+        // applies to the old window, not to that already-reduced W_max.
+        // https://www.rfc-editor.org/rfc/rfc9438.html#section-4.7
+        // https://www.rfc-editor.org/rfc/rfc9438.html#section-4.6
+        if window < self.state.w_max {
+            self.state.w_max = window * (1.0 + BETA_CUBIC) / 2.0;
         } else {
-            self.state.w_max = self.state.window as f64;
+            self.state.w_max = window;
         }
 
-        self.state.ssthresh = cmp::max(
-            (self.state.w_max * BETA_CUBIC) as u64,
-            self.minimum_window(),
-        );
+        self.state.ssthresh = cmp::max((window * BETA_CUBIC) as u64, self.minimum_window());
         self.state.window = self.state.ssthresh;
         self.state.k = self.state.cubic_k(self.current_mtu);
 
@@ -300,3 +301,26 @@ impl ControllerFactory for CubicConfig {
         Box::new(Cubic::new(self, now, current_mtu))
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn fast_convergence_reduces_w_max_without_double_reducing_window() {
+        let now = Instant::now();
+        let config = Arc::new(CubicConfig::default());
+        let mut cubic = Cubic::new(config, now, BASE_DATAGRAM_SIZE as u16);
+        let window = 8 * BASE_DATAGRAM_SIZE;
+
+        cubic.state.window = window;
+        cubic.state.ssthresh = window;
+        cubic.state.w_max = 12.0 * BASE_DATAGRAM_SIZE as f64;
+
+        cubic.on_congestion_event(now, now + Duration::from_millis(1), false, false, 0, 100);
+
+        assert_eq!(cubic.state.w_max, window as f64 * (1.0 + BETA_CUBIC) / 2.0);
+        assert_eq!(cubic.state.ssthresh, (window as f64 * BETA_CUBIC) as u64);
+        assert_eq!(cubic.state.window, cubic.state.ssthresh);
+    }
+}