Phase 8A (1/2): de-Span the serial layer, swap JSON to Newtonsoft

Prepares RioJoy.Core for the net40 (Windows XP) target, which has no
System.Memory, ValueTask, or System.Text.Json:
- IRioTransport and the whole protocol/framing layer now use byte[] +
  Task (RioPacket.Payload, PacketParser/Builder, RioChecksum, replies,
  AnalogReport, RioHidReport). At 9600 baud Span bought nothing; the
  SerialPortTransport bridge copies disappear entirely.
- ConfigStore/OverlayTemplateStore switch to Newtonsoft 13 with the
  same conventions (indented, PascalCase, string enums, null-skipping);
  verified against the real STJ-written config.json and regions.json
  (load + round-trip). System.Memory and System.Text.Json packages
  dropped.

275 tests green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/RioJoy.Core.Tests/Hid/RioHidReportTests.cs

@@ -29,12 +29,12 @@ public class RioHidReportTests
     {
         var report = new RioHidReport();
         report.SetAxis(JoyAxis.Z, 0x1234);
-        Assert.Equal(0x34, report.Bytes[4]);
-        Assert.Equal(0x12, report.Bytes[5]);
+        Assert.Equal(0x34, report.ToArray()[4]);
+        Assert.Equal(0x12, report.ToArray()[5]);
 
         report.SetAxis(JoyAxis.Z, 70000); // over max → clamped to 32767
-        Assert.Equal(0xFF, report.Bytes[4]);
-        Assert.Equal(0x7F, report.Bytes[5]);
+        Assert.Equal(0xFF, report.ToArray()[4]);
+        Assert.Equal(0x7F, report.ToArray()[5]);
     }
 
     [Theory]
@@ -47,7 +47,7 @@ public class RioHidReportTests
     {
         var report = new RioHidReport();
         report.SetHat(hat);
-        Assert.Equal(expected, report.Bytes[12] & 0x0F);
+        Assert.Equal(expected, report.ToArray()[12] & 0x0F);
     }
 
     [Theory]
@@ -59,10 +59,10 @@ public class RioHidReportTests
     {
         var report = new RioHidReport();
         report.SetButton(button, pressed: true);
-        Assert.Equal(mask, report.Bytes[byteIndex]);
+        Assert.Equal(mask, report.ToArray()[byteIndex]);
 
         report.SetButton(button, pressed: false);
-        Assert.Equal(0, report.Bytes[byteIndex]);
+        Assert.Equal(0, report.ToArray()[byteIndex]);
     }
 
     [Fact]
@@ -80,6 +80,6 @@ public class RioHidReportTests
         report.SetButton(1, true);
         report.SetButton(2, true);
         report.SetButton(1, false);
-        Assert.Equal(0x02, report.Bytes[13]); // only button 2 remains
+        Assert.Equal(0x02, report.ToArray()[13]); // only button 2 remains
     }
 }

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.AsSpan(1, packet.Length - 2).ToArray());
-        Assert.Equal(RioChecksum.Compute(packet.AsSpan(0, packet.Length - 1)), packet[^1]);
+        Assert.Equal(payload, packet.Skip(1).Take(packet.Length - 2).ToArray());
+        Assert.Equal(RioChecksum.Compute(packet, 0, packet.Length - 1), packet[packet.Length - 1]);
     }
 }

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

@@ -5,7 +5,7 @@ namespace RioJoy.Core.Tests.Protocol;
 
 public class PacketParserTests
 {
-    private static List<RioRxEvent> FeedAll(PacketParser parser, ReadOnlySpan<byte> data)
+    private static List<RioRxEvent> FeedAll(PacketParser parser, byte[] data)
     {
         var events = new List<RioRxEvent>();
         foreach (byte b in data)

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

@@ -8,7 +8,7 @@ public class RioChecksumTests
     [Fact]
     public void Empty_IsZero()
     {
-        Assert.Equal(0, RioChecksum.Compute(ReadOnlySpan<byte>.Empty));
+        Assert.Equal(0, RioChecksum.Compute(new byte[0]));
     }
 
     [Fact]

tests/RioJoy.Core.Tests/Serial/FakeTransport.cs

@@ -23,13 +23,13 @@ internal sealed class FakeTransport : IRioTransport
     /// <summary>Signal that no more inbound data will arrive (transport closed).</summary>
     public void CompleteIncoming() => _incoming.Writer.TryComplete();
 
-    public async ValueTask<int> ReadAsync(Memory<byte> buffer, CancellationToken cancellationToken)
+    public async Task<int> ReadAsync(byte[] buffer, CancellationToken cancellationToken)
     {
         while (await _incoming.Reader.WaitToReadAsync(cancellationToken).ConfigureAwait(false))
         {
             if (_incoming.Reader.TryRead(out byte[]? chunk))
             {
-                chunk.AsSpan().CopyTo(buffer.Span);
+                Array.Copy(chunk, buffer, chunk.Length);
                 return chunk.Length;
             }
         }
@@ -37,10 +37,10 @@ internal sealed class FakeTransport : IRioTransport
         return 0; // completed
     }
 
-    public ValueTask WriteAsync(ReadOnlyMemory<byte> data, CancellationToken cancellationToken)
+    public Task WriteAsync(byte[] data, CancellationToken cancellationToken)
     {
-        _writes.Writer.TryWrite(data.ToArray());
-        return default; // net48 has no ValueTask.CompletedTask; default(ValueTask) is the completed task
+        _writes.Writer.TryWrite((byte[])data.Clone());
+        return Task.CompletedTask;
     }
 
     /// <summary>Read the next outbound write, failing if none arrives in time.</summary>