Add RioSerialMonitor hardware-bringup tool

A console diagnostic that opens the RIO COM port, runs the real serial link,
and logs every event (buttons, keypad, axis, version/check, control bytes,
framing). Flashes all lamps once to prove the PC->RIO path and echoes a lamp
on each button press. Verified end-to-end against a partial RIO on COM1
(firmware 4.2; MFD + keypad inputs and the throttle axis all decode).

    dotnet run --project tools/RioSerialMonitor -- COM1 30

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/RioSerialMonitor/RioSerialMonitor.csproj

@@ -0,0 +1,15 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net8.0-windows</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <AssemblyName>RioSerialMonitor</AssemblyName>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\RioJoy.Core\RioJoy.Core.csproj" />
+  </ItemGroup>
+
+</Project>