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>

src/RioJoy.Overlay/ProfileWallpaperGenerator.cs

@@ -32,9 +32,9 @@ public sealed class ProfileWallpaperGenerator
         string outputPath,
         OverlayLayoutOptions? options = null)
     {
-        ArgumentNullException.ThrowIfNull(template);
-        ArgumentNullException.ThrowIfNull(labels);
-        ArgumentException.ThrowIfNullOrWhiteSpace(outputPath);
+        if (template is null) throw new ArgumentNullException(nameof(template));
+        if (labels is null) throw new ArgumentNullException(nameof(labels));
+        if (string.IsNullOrWhiteSpace(outputPath)) throw new ArgumentException("Value cannot be null or whitespace.", nameof(outputPath));
         if (string.IsNullOrWhiteSpace(template.BaseImagePath))
             throw new InvalidOperationException("OverlayTemplate.BaseImagePath is not set.");
 

src/RioJoy.Overlay/RioJoy.Overlay.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>
@@ -12,6 +13,10 @@
     <!-- SkiaSharp: cross-platform 2D rasterizer used to render the wallpaper. -->
     <PackageReference Include="SkiaSharp" Version="2.88.8" />
     <PackageReference Include="SkiaSharp.NativeAssets.Win32" Version="2.88.8" />
+    <PackageReference Include="PolySharp" Version="1.14.1">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
   </ItemGroup>
 
   <ItemGroup>

src/RioJoy.Overlay/SkiaOverlayRenderer.cs

@@ -35,9 +35,9 @@ public sealed class SkiaOverlayRenderer
         SKBitmap baseImage,
         OverlayLayoutOptions? options = null)
     {
-        ArgumentNullException.ThrowIfNull(template);
-        ArgumentNullException.ThrowIfNull(labels);
-        ArgumentNullException.ThrowIfNull(baseImage);
+        if (template is null) throw new ArgumentNullException(nameof(template));
+        if (labels is null) throw new ArgumentNullException(nameof(labels));
+        if (baseImage is null) throw new ArgumentNullException(nameof(baseImage));
         options ??= new OverlayLayoutOptions();
 
         var output = baseImage.Copy();
@@ -114,8 +114,8 @@ public sealed class SkiaOverlayRenderer
         string outputPath,
         OverlayLayoutOptions? options = null)
     {
-        ArgumentNullException.ThrowIfNull(template);
-        ArgumentException.ThrowIfNullOrWhiteSpace(outputPath);
+        if (template is null) throw new ArgumentNullException(nameof(template));
+        if (string.IsNullOrWhiteSpace(outputPath)) throw new ArgumentException("Value cannot be null or whitespace.", nameof(outputPath));
         if (string.IsNullOrWhiteSpace(template.BaseImagePath))
             throw new InvalidOperationException("OverlayTemplate.BaseImagePath is not set.");