receive: keep retransmissions for ACKed-but-not-yet-delivered seqs

A retransmission for a sequence number that was ACKed past (because
periodicACK crossed a gap, either in-row or via the
PktTsbpdTime <= now branch) but not yet delivered was being dropped
by the Lt(lastACKSequenceNumber) branch as "already acknowledged".

Allow retransmissions through this branch; they fall into the
out-of-order handling below where they either fill the gap or are
detected as duplicates. TLPKTDROP semantics remain enforced by the
earlier Lte(lastDeliveredSequenceNumber) check, which still drops
anything already delivered or skipped past.

Makes the regression test from the previous commit pass.

Made-with: Cursor
Co-authored-by: Cursor <cursoragent@cursor.com>

Author: JohanG-LAS

congestion/live/receive.go

@@ -194,11 +194,24 @@ func (r *receiver) Push(pkt packet.Packet) {
 	}
 
 	if pkt.Header().PacketSequenceNumber.Lt(r.lastACKSequenceNumber) {
-		// Already acknowledged, ignoring
-		r.statistics.PktDrop++
-		r.statistics.ByteDrop += pktLen
+		if !pkt.Header().RetransmittedPacketFlag {
+			// Already acknowledged, ignoring
+			r.statistics.PktDrop++
+			r.statistics.ByteDrop += pktLen
 
-		return
+			return
+		}
+		// Retransmission for a sequence number that was ACKed past but
+		// not yet delivered. periodicACK and the delivery loop are
+		// separate critical sections within Tick: lastACKSequenceNumber
+		// can advance past a gap (e.g. when packets after the gap have
+		// ripe PktTsbpdTime) before lastDeliveredSequenceNumber catches
+		// up. A retransmission may legitimately arrive in that window.
+		// Allow it to flow into the out-of-order branch below, where
+		// it will either fill the gap or be detected as a duplicate.
+		// TLPKTDROP semantics are still enforced by the earlier
+		// Lte(lastDeliveredSequenceNumber) check, which drops anything
+		// already delivered or skipped past.
 	}
 
 	if pkt.Header().PacketSequenceNumber.Equals(r.maxSeenSequenceNumber.Inc()) {