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/Profiles/ConfigStore.cs

@@ -1,32 +1,36 @@
-using System.Text.Json;
-using System.Text.Json.Serialization;
+using Newtonsoft.Json;
+using Newtonsoft.Json.Converters;
 
 namespace RioJoy.Core.Profiles;
 
 /// <summary>
 /// Loads and saves <see cref="AppConfig"/> as JSON. Replaces the legacy
 /// SimpleIni single-file config with a profile library (see docs/PLAN.md).
+/// Newtonsoft rather than System.Text.Json so the net40 (Windows XP) flavor
+/// shares one serializer — and one on-disk format — with net48 (PLAN.md §8A).
+/// Conventions match the original STJ settings: indented, PascalCase names,
+/// enums as strings, nulls skipped — existing config.json files parse as-is.
 /// </summary>
 public static class ConfigStore
 {
-    private static readonly JsonSerializerOptions Options = new()
+    private static readonly JsonSerializerSettings Options = new()
     {
-        WriteIndented = true,
-        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
-        Converters = { new JsonStringEnumConverter() },
+        Formatting = Formatting.Indented,
+        NullValueHandling = NullValueHandling.Ignore,
+        Converters = { new StringEnumConverter() },
     };
 
     public static string Serialize(AppConfig config)
     {
         if (config is null) throw new ArgumentNullException(nameof(config));
-        return JsonSerializer.Serialize(config, Options);
+        return JsonConvert.SerializeObject(config, Options);
     }
 
     public static AppConfig Deserialize(string json)
     {
         if (string.IsNullOrWhiteSpace(json)) throw new ArgumentException("Value cannot be null or whitespace.", nameof(json));
-        return JsonSerializer.Deserialize<AppConfig>(json, Options)
-               ?? throw new JsonException("Config JSON deserialized to null.");
+        return JsonConvert.DeserializeObject<AppConfig>(json, Options)
+               ?? throw new JsonSerializationException("Config JSON deserialized to null.");
     }
 
     /// <summary>Save the config to <paramref name="path"/> (creating directories).</summary>