serial: pipe:vrio named-pipe transport, no com0com needed

NamedPipeTransport connects as a client to vRIO's \\.\pipe\vrio (the
DOSBox-X fork's role) and speaks the shared typed-frame contract
(PipeFraming: 0x00 data / 0x01 modem lines, null-modem crossed). The COM
path's DTR reset pulse is replayed in-band on connect. Peer disconnects
and framing violations surface as the 0-byte transport-closed read the
link already understands.

RioTransportFactory routes endpoint strings — pipe:name (vRIO's own
picker syntax) to the pipe transport, everything else to
SerialPortTransport — and is wired into RioCoordinator and all three
RioSerialMonitor modes, so profiles (RioComPort/DefaultRioComPort) and
the bench tools take pipe endpoints anywhere a COM name went.

Gotcha baked into the design: named pipes here have 0-byte buffers, so a
write blocks until the peer reads it, and vRIO also writes its lines
frame before reading — the on-connect pulse frames are therefore queued
as overlapped writes (pipe writes drain in issue order, preserving the
edge positions) instead of blocking the constructor into a mutual
write-first deadlock.

Verified end-to-end against the real VRioDevice + VRioPipeService over
\\.\pipe\vrio: version 4.2 + check replies, 137 analog polls, lamp
commands ACKed, zero framing errors. 322 tests green, both flavors.

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

tools/RioSerialMonitor/E0Test.cs

@@ -70,7 +70,7 @@ internal static class E0Test
         int frameCountdown = 0;    // >0: inside a reply frame, bytes remaining
         bool ackSent = false;
 
-        SerialPortTransport? transport = null;
+        IRioTransport? transport = null;
         Task? reader = null;
 
         void PhaseReset(AckMode mode, int expectReply, int expectLen)
@@ -93,7 +93,7 @@ internal static class E0Test
         foreach (int tryBaud in bauds)
         {
             Console.WriteLine($"[{sw.Elapsed.TotalSeconds,6:F2}s] probing {port} @ {tryBaud} (DTR reset, ~2s boot wait)...");
-            try { transport = new SerialPortTransport(port, tryBaud); }
+            try { transport = RioTransportFactory.Open(port, tryBaud); }
             catch (Exception ex)
             {
                 Console.WriteLine($"  FAILED to open {port}: {ex.GetType().Name}: {ex.Message}");
@@ -101,7 +101,7 @@ internal static class E0Test
             }
 
             lock (gate) { buffer.Clear(); cursor = 0; }
-            SerialPortTransport t = transport;
+            IRioTransport t = transport;
             reader = Task.Run(async () =>
             {
                 var tmp = new byte[256];