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/Output/HidFeederJoystickSink.cs

@@ -1,5 +1,4 @@
 using System.Runtime.InteropServices;
-using System.Runtime.Versioning;
 using Microsoft.Win32.SafeHandles;
 using RioJoy.Core.Calibration;
 using RioJoy.Core.Hid;
@@ -14,7 +13,6 @@ namespace RioJoy.Core.Output;
 /// <c>DeviceIoControl(IOCTL_RIO_SUBMIT_REPORT)</c>. Replaces
 /// <see cref="NullJoystickSink"/> once the driver is installed (Phase 1/3b).
 /// </summary>
-[SupportedOSPlatform("windows")]
 public sealed class HidFeederJoystickSink : IJoystickSink, IDisposable
 {
     // Must match driver/RioGamepad/Public.h.

src/RioJoy.Core/Output/SendInputSink.cs

@@ -1,5 +1,4 @@
 using System.Runtime.InteropServices;
-using System.Runtime.Versioning;
 using RioJoy.Core.Mapping;
 using MouseButtonId = RioJoy.Core.Mapping.MouseButton;
 
@@ -12,7 +11,6 @@ namespace RioJoy.Core.Output;
 /// scancodes (most do) see the keypress. Targets the interactive session, which
 /// is why RIOJoy is a tray app rather than a service (see docs/PLAN.md risks).
 /// </summary>
-[SupportedOSPlatform("windows")]
 public sealed class SendInputSink : IInputSink
 {
     public void KeyDown(byte virtualKey, bool extended) => SendKey(virtualKey, extended, keyUp: false);

src/RioJoy.Core/Output/ViGEmJoystickSink.cs

@@ -1,4 +1,3 @@
-using System.Runtime.Versioning;
 using Nefarius.ViGEm.Client;
 using Nefarius.ViGEm.Client.Targets;
 using Nefarius.ViGEm.Client.Targets.Xbox360;
@@ -14,7 +13,6 @@ namespace RioJoy.Core.Output;
 /// triggers, and the D-pad for the hat. RIO joystick buttons beyond
 /// <see cref="MappableButtonCount"/> are dropped (map those to the keyboard).
 /// </summary>
-[SupportedOSPlatform("windows")]
 public sealed class ViGEmJoystickSink : IJoystickSink, IDisposable
 {
     // RIO joystick button (1-based) -> Xbox 360 button, in a natural order.
@@ -115,11 +113,11 @@ public sealed class ViGEmJoystickSink : IJoystickSink, IDisposable
 
     // RIO axis 0..32766 (centre 16383) -> Xbox thumb short -32768..32767.
     private static short ToThumb(int value) =>
-        (short)Math.Clamp((value - AxisOutputs.Center) * 2, short.MinValue, short.MaxValue);
+        (short)RioJoy.Core.Compat.Net48Math.Clamp((value - AxisOutputs.Center) * 2, short.MinValue, short.MaxValue);
 
     // RIO axis 0..32766 -> Xbox trigger byte 0..255.
     private static byte ToTrigger(int value) =>
-        (byte)Math.Clamp(value * 255 / AxisOutputs.Max, 0, 255);
+        (byte)RioJoy.Core.Compat.Net48Math.Clamp(value * 255 / AxisOutputs.Max, 0, 255);
 
     public void Dispose()
     {