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>

src/RioJoy.Core/Serial/IRioTransport.cs

@@ -15,8 +15,13 @@ public interface IRioTransport : IDisposable
     /// Read available bytes into <paramref name="buffer"/>. Returns the number of
     /// bytes read; a return of 0 indicates the transport has closed.
     /// </summary>
-    ValueTask<int> ReadAsync(Memory<byte> buffer, CancellationToken cancellationToken);
+    /// <remarks>
+    /// Plain <c>byte[]</c>/<see cref="Task"/> rather than Memory/ValueTask: the
+    /// link runs at 9600 baud, and the net40 (Windows XP) flavor has neither
+    /// System.Memory nor ValueTask (see docs/PLAN.md §Phase 8).
+    /// </remarks>
+    Task<int> ReadAsync(byte[] buffer, CancellationToken cancellationToken);
 
     /// <summary>Write all of <paramref name="data"/> to the transport.</summary>
-    ValueTask WriteAsync(ReadOnlyMemory<byte> data, CancellationToken cancellationToken);
+    Task WriteAsync(byte[] data, CancellationToken cancellationToken);
 }

src/RioJoy.Core/Serial/RioSerialLink.cs

@@ -79,7 +79,7 @@ public sealed class RioSerialLink
     }
 
     /// <summary>Send a pre-built packet (see <see cref="PacketBuilder"/>) to the RIO.</summary>
-    public async Task SendAsync(ReadOnlyMemory<byte> packet, CancellationToken cancellationToken = default)
+    public async Task SendAsync(byte[] packet, CancellationToken cancellationToken = default)
     {
         await _writeLock.WaitAsync(cancellationToken).ConfigureAwait(false);
         try
@@ -155,7 +155,7 @@ public sealed class RioSerialLink
         switch (packet.Command)
         {
             case RioCommand.AnalogReply:
-                if (AnalogReport.TryParse(packet.Payload.Span, out AnalogReport report))
+                if (AnalogReport.TryParse(packet.Payload, out AnalogReport report))
                 {
                     _sinceAnalog.Restart();
                     AnalogReceived?.Invoke(report);
@@ -163,11 +163,11 @@ public sealed class RioSerialLink
                 break;
 
             case RioCommand.VersionReply:
-                VersionReceived?.Invoke(VersionInfo.Parse(packet.Payload.Span));
+                VersionReceived?.Invoke(VersionInfo.Parse(packet.Payload));
                 break;
 
             case RioCommand.CheckReply:
-                CheckReceived?.Invoke(CheckStatus.Parse(packet.Payload.Span));
+                CheckReceived?.Invoke(CheckStatus.Parse(packet.Payload));
                 break;
         }
     }

src/RioJoy.Core/Serial/SerialPortTransport.cs

@@ -47,21 +47,11 @@ public sealed class SerialPortTransport : IRioTransport
 
     public string Description => $"{_port.PortName} @ {BaudRate} 8N1";
 
-    // net48's Stream has no Memory-based ReadAsync/WriteAsync overloads, so bridge
-    // through a pooled array and copy into/out of the caller's Memory<byte>.
-    public async ValueTask<int> ReadAsync(Memory<byte> buffer, CancellationToken cancellationToken)
-    {
-        byte[] tmp = new byte[buffer.Length];
-        int read = await _stream.ReadAsync(tmp, 0, tmp.Length, cancellationToken).ConfigureAwait(false);
-        new ReadOnlySpan<byte>(tmp, 0, read).CopyTo(buffer.Span);
-        return read;
-    }
+    public Task<int> ReadAsync(byte[] buffer, CancellationToken cancellationToken) =>
+        _stream.ReadAsync(buffer, 0, buffer.Length, cancellationToken);
 
-    public async ValueTask WriteAsync(ReadOnlyMemory<byte> data, CancellationToken cancellationToken)
-    {
-        byte[] tmp = data.ToArray();
-        await _stream.WriteAsync(tmp, 0, tmp.Length, cancellationToken).ConfigureAwait(false);
-    }
+    public Task WriteAsync(byte[] data, CancellationToken cancellationToken) =>
+        _stream.WriteAsync(data, 0, data.Length, cancellationToken);
 
     public void Dispose()
     {