net48 port (test branch): retarget all projects to .NET Framework 4.8

Retargets RioJoy.Core/Overlay/Tray + tests from net8.0-windows to net48 so
the app can be tested as a framework-dependent build (relies on the in-box
.NET Framework 4.8 on Windows 10/11). Builds clean; all 241 tests pass.

Polyfills (no behavior change):
- PolySharp source generator for init/records/Index/Range/required members.
- System.Memory, System.Text.Json, Microsoft.Bcl.HashCode,
  System.Threading.Channels (tests) NuGet packages.
- Compat/Net48Polyfills.cs: GetValueOrDefault, KeyValuePair.Deconstruct,
  Math.Clamp; tests/TestPolyfills.cs: Task.WaitAsync.

Source adjustments for APIs absent on net48:
- ArgumentNullException/ArgumentException.ThrowIf* inlined to manual guards.
- Convert.ToHexString, Encoding.Latin1, Environment.ProcessPath,
  ApplicationConfiguration.Initialize, Enum.GetNames<T>/GetValues<T>,
  string.StartsWith(char), string.Split(char, opts), TextBox.PlaceholderText,
  PeriodicTimer, Memory-based Stream Read/WriteAsync, array range-slicing.
- Dropped [SupportedOSPlatform] hints (net48 is single-platform).

deploy/build-package.ps1: publish framework-dependent (no self-contained).

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

tests/RioJoy.Core.Tests/Overlay/SkiaOverlayRendererTests.cs

@@ -77,6 +77,6 @@ public class SkiaOverlayRendererTests
         byte[] png = new SkiaOverlayRenderer().RenderToPng(template, new Dictionary<string, string> { ["x"] = "HI" }, baseImg);
 
         Assert.True(png.Length > 8);
-        Assert.Equal(new byte[] { 0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A }, png[..8]);
+        Assert.Equal(new byte[] { 0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A }, png.AsSpan(0, 8).ToArray());
     }
 }

tests/RioJoy.Core.Tests/Protocol/PacketBuilderTests.cs

@@ -41,7 +41,7 @@ public class PacketBuilderTests
         byte[] packet = PacketBuilder.Build(RioCommand.AnalogReply, payload);
 
         Assert.Equal((byte)RioCommand.AnalogReply, packet[0]);
-        Assert.Equal(payload, packet[1..^1]);
-        Assert.Equal(RioChecksum.Compute(packet[..^1]), packet[^1]);
+        Assert.Equal(payload, packet.AsSpan(1, packet.Length - 2).ToArray());
+        Assert.Equal(RioChecksum.Compute(packet.AsSpan(0, packet.Length - 1)), packet[^1]);
     }
 }

tests/RioJoy.Core.Tests/RioJoy.Core.Tests.csproj

@@ -1,8 +1,9 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net8.0-windows</TargetFramework>
+    <TargetFramework>net48</TargetFramework>
     <Platforms>x64</Platforms>
+    <PlatformTarget>x64</PlatformTarget>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
     <LangVersion>latest</LangVersion>
@@ -13,6 +14,11 @@
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.11.1" />
     <PackageReference Include="xunit" Version="2.9.2" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2" />
+    <PackageReference Include="System.Threading.Channels" Version="8.0.0" />
+    <PackageReference Include="PolySharp" Version="1.14.1">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
   </ItemGroup>
 
   <ItemGroup>

tests/RioJoy.Core.Tests/Serial/FakeTransport.cs

@@ -40,7 +40,7 @@ internal sealed class FakeTransport : IRioTransport
     public ValueTask WriteAsync(ReadOnlyMemory<byte> data, CancellationToken cancellationToken)
     {
         _writes.Writer.TryWrite(data.ToArray());
-        return ValueTask.CompletedTask;
+        return default; // net48 has no ValueTask.CompletedTask; default(ValueTask) is the completed task
     }
 
     /// <summary>Read the next outbound write, failing if none arrives in time.</summary>

tests/RioJoy.Core.Tests/TestPolyfills.cs

@@ -0,0 +1,21 @@
+namespace RioJoy.Core.Tests;
+
+/// <summary>
+/// net48 test-only polyfills. Visible to all tests via enclosing-namespace scope.
+/// </summary>
+internal static class TaskTestExtensions
+{
+    /// <summary>
+    /// Polyfill for Task&lt;T&gt;.WaitAsync(TimeSpan) (net6+), absent on net48.
+    /// Throws <see cref="TimeoutException"/> if the task does not complete in time.
+    /// </summary>
+    public static async Task<T> WaitAsync<T>(this Task<T> task, TimeSpan timeout)
+    {
+        using var cts = new CancellationTokenSource();
+        Task completed = await Task.WhenAny(task, Task.Delay(timeout, cts.Token)).ConfigureAwait(false);
+        if (completed != task)
+            throw new TimeoutException($"Task did not complete within {timeout}.");
+        cts.Cancel(); // stop the delay timer
+        return await task.ConfigureAwait(false);
+    }
+}