RioSerialLink: NAK-driven retransmit of the last command packet

The board ACK/NAKs every inbound packet; we were fire-and-forget, so
any corrupt arrival became a permanent state error (stuck-bright /
missed lamps at 62500, where ~6.5% of 4-byte lamp commands lose bytes
to RX-ISR overrun). Now a NAK control byte triggers a resend of the
most recent command packet, bounded by NakRetransmitLimit (default 2;
0 restores fire-and-forget).

Design notes: the wire has no sequence numbers, but every PC->RIO
command is idempotent (lamp state, analog request, reset), so resending
the latest command is safe even in the rare race where the NAK belonged
to an earlier packet. Single control-byte replies (our ACKs) never
participate. NakRetransmits counter surfaced in the mash summary.

6 new tests (retransmit, limit, budget reset, ACK no-op, control-byte
exclusion, disable switch); 281 green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tools/RioSerialMonitor/MashTest.cs

@@ -319,7 +319,7 @@ internal static class MashTest
         Raw($"firmware         : {(version is null ? "(no reply)" : version.ToString())}");
         Raw($"presses/releases : {presses} / {releases}  ({presses / mins:F0} presses/min)");
         Raw($"analog replies   : {analogReplies} (~{expectedPolls} poll slots; {100.0 * analogReplies / Math.Max(1, expectedPolls):F1}%), sentinels {sentinels}");
-        Raw($"framing / NAK    : {framing} / {naks}");
+        Raw($"framing / NAK    : {framing} / {naks} (NAK-triggered resends: {link.NakRetransmits})");
         Raw("gap histogram    : " + string.Join("  ", bucketNames.Select((n, i) => $"{n}:{buckets[i]}")));
         Raw("longest gaps     : " + (longestGaps.Count == 0
             ? "(none)"