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/Protocol/PacketParser.cs

@@ -92,15 +92,17 @@ public sealed class PacketParser
     }
 
     /// <summary>
-    /// Feed a chunk of received bytes, invoking <paramref name="onEvent"/> for
-    /// each event produced, in order.
+    /// Feed the first <paramref name="count"/> received bytes of
+    /// <paramref name="data"/>, invoking <paramref name="onEvent"/> for each
+    /// event produced, in order.
     /// </summary>
-    public void Feed(ReadOnlySpan<byte> data, Action<RioRxEvent> onEvent)
+    public void Feed(byte[] data, int count, Action<RioRxEvent> onEvent)
     {
+        if (data is null) throw new ArgumentNullException(nameof(data));
         if (onEvent is null) throw new ArgumentNullException(nameof(onEvent));
-        foreach (byte ch in data)
+        for (int i = 0; i < count; i++)
         {
-            if (Feed(ch, out RioRxEvent ev))
+            if (Feed(data[i], out RioRxEvent ev))
                 onEvent(ev);
         }
     }
@@ -110,11 +112,12 @@ public sealed class PacketParser
         // _buffer = [command][payload…][checksum]; _count includes all of them.
         int bodyLen = _count - 1;             // command + payload (exclude checksum)
         byte receivedChecksum = _buffer[bodyLen];
-        byte computed = RioChecksum.Compute(_buffer.AsSpan(0, bodyLen));
+        byte computed = RioChecksum.Compute(_buffer, 0, bodyLen);
         bool checksumValid = computed == receivedChecksum;
 
         var command = (RioCommand)_buffer[0];
-        var payload = _buffer.AsSpan(1, bodyLen - 1).ToArray(); // copy out; buffer is reused
+        var payload = new byte[bodyLen - 1];  // copy out; buffer is reused
+        Array.Copy(_buffer, 1, payload, 0, payload.Length);
         var packet = new RioPacket(command, payload);
 
         Reset();