fix: a full reorder ring keeps the arriving packet and counts the overflow, instead of dropping it silently

A packet that needed a slot and found none was neither held nor delivered: no
counter recorded it and the consumer reserved a hole nothing could ever backfill.
It now commits the gap to free a slot and takes the ordering decision once more
against the sequence that commit left behind, bounded to one retry; past that it
is delivered in order with the counters it skipped as its `lost`, never flagged
late, because a packet ahead of the sequence has no hole to be looked up by. A
duplicate can now replace its slot at a full ring, as hold_packet's own comment
already promised, and the 20 ms default window is a named constant.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/acquisition.cpp

@@ -94,6 +94,7 @@ void Acquisition::run() {
     out.resequenced.store(s.late_resequenced, std::memory_order_relaxed);
     out.late.store(s.late_delivered, std::memory_order_relaxed);
     out.stale.store(s.stale_packets, std::memory_order_relaxed);
+    out.overflows.store(s.reorder_overflows, std::memory_order_relaxed);
     out.reconnects.store(s.reconnects, std::memory_order_relaxed);
     out.connected.store(udps_client_is_connected(client_) != 0, std::memory_order_relaxed);
   }