feat: hold a packet that runs ahead of the sequence, and deliver a late one flagged

deps/udpsclient/udps_client.h

@@ -150,6 +150,13 @@ typedef struct {
      * range the next frame claims.
      */
     uint32_t lost;
+    /**
+     * 1 when this frame's counter is behind the sequence already delivered: it lost
+     * the race past the reorder window and the consumer has a hole where it belongs.
+     * @c lost is 0 for such a frame -- it is not missing anything, it *is* what was
+     * missing -- so a consumer sizes its backfill from @c counter, not from @c lost.
+     */
+    uint8_t  late;
     uint64_t hrt;          /**< Producer's high-resolution timer at send. */
     double   recv_time;    /**< Wall-clock seconds (CLOCK_REALTIME) at arrival. */
     uint8_t  publish_mode; /**< UDPS_PUBLISH_*. */
@@ -180,6 +187,17 @@ typedef struct {
      * time base older than data already handed over.
      */
     uint64_t stale_packets;
+    /**
+     * DATA packets that arrived out of order and were put *back* in order inside
+     * the reorder window, so the consumer never saw a gap for them.
+     */
+    uint64_t late_resequenced;
+    /**
+     * DATA packets delivered behind the sequence because the reorder window had
+     * already closed on them. The consumer has a hole for these and may fill it;
+     * before the window existed they were counted in @c stale_packets and dropped.
+     */
+    uint64_t late_delivered;
     uint64_t reconnects;
 } udps_stats_t;
 
@@ -226,6 +244,14 @@ typedef struct {
     double      keepalive_interval_s;/**< Unicast ACK period. 0 disables. */
     uint32_t    recv_buffer_bytes;   /**< SO_RCVBUF; large bursts need a large value. */
     uint32_t    max_packet_bytes;    /**< Ceiling on one reassembled payload. */
+    /**
+     * How long a packet may be held waiting for an earlier counter, in seconds.
+     *
+     * A *contiguous* arrival is never held, so this is the latency a gap costs and
+     * not a latency the stream pays. 0 disables resequencing entirely and restores
+     * the drop-on-late behaviour. Default 0.020.
+     */
+    double      reorder_window_s;
 } udps_client_config_t;
 
 /** Fills @p cfg with the defaults documented in Docs/UDPS-C-Client.md. */