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/Hid/RioHidReport.cs

@@ -52,7 +52,7 @@ public sealed class RioHidReport
     /// <summary>Set an axis value (clamped to 0..<see cref="AxisMax"/>), little-endian.</summary>
     public void SetAxis(JoyAxis axis, int value)
     {
-        ushort v = (ushort)Math.Clamp(value, 0, AxisMax);
+        ushort v = (ushort)RioJoy.Core.Compat.Net48Math.Clamp(value, 0, AxisMax);
         int offset = (int)axis * 2;
         _buffer[offset] = (byte)(v & 0xFF);
         _buffer[offset + 1] = (byte)(v >> 8);