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>

tests/RioJoy.Core.Tests/Protocol/PacketBuilderTests.cs

@@ -41,7 +41,7 @@ public class PacketBuilderTests
         byte[] packet = PacketBuilder.Build(RioCommand.AnalogReply, payload);
 
         Assert.Equal((byte)RioCommand.AnalogReply, packet[0]);
-        Assert.Equal(payload, packet[1..^1]);
-        Assert.Equal(RioChecksum.Compute(packet[..^1]), packet[^1]);
+        Assert.Equal(payload, packet.AsSpan(1, packet.Length - 2).ToArray());
+        Assert.Equal(RioChecksum.Compute(packet.AsSpan(0, packet.Length - 1)), packet[^1]);
     }
 }