Docs: reflect net48; convert tools projects to net48

Documentation now describes the .NET Framework 4.8 stack (was .NET 8):
- README build prerequisites + framework-dependent/no-runtime-install note,
  test count 136 -> 241.
- PLAN.md architecture diagram, stack decision (with PolySharp/shims note),
  suite total 136 -> 241.
- deploy/README-DEPLOY.txt: app is net48 framework-dependent (4.8 is in-box on
  Win10/11), and build prerequisites.

Also migrate the three tools (RioJoySmokeTest, RioSerialMonitor, XcfRegionExtract)
to net48 so they keep building against the now-net48 RioJoy.Core (a net8 project
cannot reference a net48 one). Added PolySharp + System.Memory/System.Text.Json
shims and replaced net-core-only APIs (string.Contains/IndexOf with comparison,
Array.Fill, generic Enum.IsDefined, int.TryParse(span)). All three build clean.

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

tools/RioJoySmokeTest/Program.cs

@@ -105,7 +105,7 @@ int FindJoyId()
         if (firstPresent < 0) { firstPresent = id; lastName = name; }
         // RioGamepad identity (Public.h: RIO_VENDOR_ID 0x1209 / RIO_PRODUCT_ID 0x5249).
         if (caps.wMid == 0x1209 && caps.wPid == 0x5249) { lastName = name; return id; }
-        if (name.Contains("RIOJoy", StringComparison.OrdinalIgnoreCase)) { lastName = name; return id; }
+        if (name.IndexOf("RIOJoy", StringComparison.OrdinalIgnoreCase) >= 0) { lastName = name; return id; }
     }
     return firstPresent;
 }

tools/RioJoySmokeTest/RioJoySmokeTest.csproj

@@ -1,9 +1,10 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net8.0-windows</TargetFramework>
+    <TargetFramework>net48</TargetFramework>
     <Nullable>enable</Nullable>
     <ImplicitUsings>enable</ImplicitUsings>
+    <LangVersion>latest</LangVersion>
     <Platforms>x64</Platforms>
     <PlatformTarget>x64</PlatformTarget>
     <AssemblyName>RioJoySmokeTest</AssemblyName>
@@ -12,4 +13,10 @@
   <ItemGroup>
     <ProjectReference Include="..\..\src\RioJoy.Core\RioJoy.Core.csproj" />
   </ItemGroup>
+  <ItemGroup>
+    <PackageReference Include="PolySharp" Version="1.14.1">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
+  </ItemGroup>
 </Project>