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/IniFile.cs

@@ -11,7 +11,7 @@ public sealed class IniFile
 
     public static IniFile Parse(string text)
     {
-        ArgumentNullException.ThrowIfNull(text);
+        if (text is null) throw new ArgumentNullException(nameof(text));
         var ini = new IniFile();
         var current = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
         ini._sections[string.Empty] = current;
@@ -68,9 +68,9 @@ public sealed class IniFile
     /// <summary>Parse an integer value that may be hex (<c>0x…</c>) or decimal.</summary>
     public static int ParseInt(string value)
     {
-        ArgumentNullException.ThrowIfNull(value);
+        if (value is null) throw new ArgumentNullException(nameof(value));
         string v = value.Trim();
-        bool neg = v.StartsWith('-');
+        bool neg = v.StartsWith("-", StringComparison.Ordinal);
         if (neg) v = v[1..];
 
         int magnitude = v.StartsWith("0x", StringComparison.OrdinalIgnoreCase)