Firmware: RIOv4_2_patched_31250.bin — wedge fix + SCI retuned to 31250 baud

make_patch.py gains --baud31250: one byte beyond the wedge patch — the SCI
init operand at $D62B ($30 -> $02). BAUD $30 = /13 prescale, /1 divider
(2MHz E / 208 = 9615); $02 = /1, /4 -> 31250 exactly, 3.3x faster. The
init write also confirms the 8MHz crystal, which is why 19200/38400 are
unreachable and why 62500/125k are left alone pending an ISR cycle count.

- 24 bytes changed vs original (sha256 9f866cf3...); re-disassembly diff
  vs the classic patched image shows exactly the one operand line, and
  the classic build still reproduces 3fc8170c... (script regression-safe).
- PC side: SerialPortTransport takes an optional baudRate (default 9600,
  runtime untouched); RioSerialMonitor + --mash accept --baud 31250.
- Caveats documented in README/ANALYSIS: non-standard rate (FTDI-class
  adapters only, no 16550s); native games still speak 9600 so this chip
  is bench/RIOJoy-only; validate the wedge patch at 9600 first.

275 tests green; mash --selftest regression unchanged (FAIL/exit-1).

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

src/RioJoy.Core/Serial/SerialPortTransport.cs

@@ -11,7 +11,7 @@ namespace RioJoy.Core.Serial;
 /// </summary>
 public sealed class SerialPortTransport : IRioTransport
 {
-    /// <summary>RIO link bit rate.</summary>
+    /// <summary>Default RIO link bit rate (stock v4.2 firmware).</summary>
     public const int BaudRate = 9600;
 
     /// <summary>DTR reset-pulse hold time on open.</summary>
@@ -23,11 +23,18 @@ public sealed class SerialPortTransport : IRioTransport
     private readonly SerialPort _port;
     private readonly Stream _stream;
 
-    public SerialPortTransport(string portName)
+    /// <param name="portName">COM port name, e.g. "COM1".</param>
+    /// <param name="baudRate">
+    /// Link rate; default 9600 (stock firmware). The 31250-baud firmware
+    /// variant (rio-firmware/, --baud31250) needs an adapter that can make
+    /// non-standard rates exactly — FTDI-class USB serial can, classic
+    /// 16550 UARTs cannot.
+    /// </param>
+    public SerialPortTransport(string portName, int baudRate = BaudRate)
     {
         if (string.IsNullOrWhiteSpace(portName)) throw new ArgumentException("Value cannot be null or whitespace.", nameof(portName));
 
-        _port = new SerialPort(portName, BaudRate, Parity.None, 8, StopBits.One)
+        _port = new SerialPort(portName, baudRate, Parity.None, 8, StopBits.One)
         {
             Handshake = Handshake.None,
             // The framing state machine handles timing; keep reads non-throwing.
@@ -45,7 +52,7 @@ public sealed class SerialPortTransport : IRioTransport
         _stream = _port.BaseStream;
     }
 
-    public string Description => $"{_port.PortName} @ {BaudRate} 8N1";
+    public string Description => $"{_port.PortName} @ {_port.BaudRate} 8N1";
 
     public Task<int> ReadAsync(byte[] buffer, CancellationToken cancellationToken) =>
         _stream.ReadAsync(buffer, 0, buffer.Length, cancellationToken);