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.Core/Profiles/ConfigStore.cs

@@ -18,13 +18,13 @@ public static class ConfigStore
 
     public static string Serialize(AppConfig config)
     {
-        ArgumentNullException.ThrowIfNull(config);
+        if (config is null) throw new ArgumentNullException(nameof(config));
         return JsonSerializer.Serialize(config, Options);
     }
 
     public static AppConfig Deserialize(string json)
     {
-        ArgumentException.ThrowIfNullOrWhiteSpace(json);
+        if (string.IsNullOrWhiteSpace(json)) throw new ArgumentException("Value cannot be null or whitespace.", nameof(json));
         return JsonSerializer.Deserialize<AppConfig>(json, Options)
                ?? throw new JsonException("Config JSON deserialized to null.");
     }
@@ -32,7 +32,7 @@ public static class ConfigStore
     /// <summary>Save the config to <paramref name="path"/> (creating directories).</summary>
     public static void Save(AppConfig config, string path)
     {
-        ArgumentException.ThrowIfNullOrWhiteSpace(path);
+        if (string.IsNullOrWhiteSpace(path)) throw new ArgumentException("Value cannot be null or whitespace.", nameof(path));
         string? dir = Path.GetDirectoryName(Path.GetFullPath(path));
         if (!string.IsNullOrEmpty(dir))
             Directory.CreateDirectory(dir);
@@ -45,7 +45,7 @@ public static class ConfigStore
     /// </summary>
     public static AppConfig Load(string path)
     {
-        ArgumentException.ThrowIfNullOrWhiteSpace(path);
+        if (string.IsNullOrWhiteSpace(path)) throw new ArgumentException("Value cannot be null or whitespace.", nameof(path));
         return File.Exists(path) ? Deserialize(File.ReadAllText(path)) : new AppConfig();
     }
 }