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>

deploy/build-package.ps1

@@ -47,12 +47,12 @@ try {
     $pkgDir = Join-Path $staging 'RIOJoy'
     New-Item -ItemType Directory -Force -Path $pkgDir | Out-Null
 
-    # 1. Publish the tray app into riojoy\app (self-contained win-x64 — no .NET on target).
+    # 1. Publish the tray app into riojoy\app (net48, framework-dependent — relies on
+    #    the in-box .NET Framework 4.8 present on every Windows 10/11 machine).
     $appOut = Join-Path $pkgDir 'app'
-    Write-Host "Publishing RioJoy.Tray ($Configuration, self-contained win-x64)..."
+    Write-Host "Publishing RioJoy.Tray ($Configuration, net48 framework-dependent)..."
     & dotnet publish (Join-Path $repo 'src\RioJoy.Tray\RioJoy.Tray.csproj') `
-        -c $Configuration -r win-x64 --self-contained true `
-        -p:PublishSingleFile=false -p:DebugType=none `
+        -c $Configuration -p:DebugType=none `
         -o $appOut | Out-Null
     if ($LASTEXITCODE -ne 0) { throw 'dotnet publish failed.' }