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>

tools/RioSerialMonitor/MashTest.cs

@@ -51,6 +51,7 @@ internal static class MashTest
         bool lampEcho = true;
         double wedgeSec = 2.0;
         bool selftest = false;
+        int baud = 9600;
 
         var positional = new List<string>();
         for (int i = 0; i < args.Length; i++)
@@ -63,6 +64,8 @@ internal static class MashTest
                 case "--label" when i + 1 < args.Length: label = args[++i]; break;
                 case "--wedge" when i + 1 < args.Length && double.TryParse(args[i + 1], out double w):
                     wedgeSec = w; i++; break;
+                case "--baud" when i + 1 < args.Length && int.TryParse(args[i + 1], out int b):
+                    baud = b; i++; break;
                 default: positional.Add(args[i]); break;
             }
         }
@@ -96,7 +99,7 @@ internal static class MashTest
             lock (gate) { Console.WriteLine(msg); logFile.WriteLine(msg); }
         }
 
-        Raw($"== RIO mash test :: {port} @ 9600 8N1, {seconds}s, chip label '{label}' ==");
+        Raw($"== RIO mash test :: {port} @ {baud} 8N1, {seconds}s, chip label '{label}' ==");
         Raw($"   lamp echo {(lampEcho ? "ON (drives reply/lamp collisions)" : "OFF")}, " +
             $"wedge threshold {wedgeSec:F1}s, app auto-recovery DISABLED");
         Raw($"   log: {Path.GetFullPath(logPath)}");
@@ -104,7 +107,7 @@ internal static class MashTest
         IRioTransport transport;
         try
         {
-            transport = selftest ? new SelftestTransport() : new SerialPortTransport(port);
+            transport = selftest ? new SelftestTransport() : new SerialPortTransport(port, baud);
         }
         catch (Exception ex)
         {