Invert Y on the wire, not in the router: the panel keeps tracking the stick

InputRouter.InvertJoystickY flipped the stored axis, so the X/Y dot and
readout moved opposite the physical stick. The toggle now lives on
VRioDevice and negates joystick Y only while building AnalogReply: local
state keeps the physical direction for every source (pad, keys, panel
drags) and only the host sees the flip. Negating a full -8192 deflection
lands one past the 14-bit Max, so the reply clamps it to 8191.

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

src/VRio.Core/Device/VRioDevice.cs

@@ -57,6 +57,13 @@ public sealed class VRioDevice
     /// <summary>Firmware version reported by VersionReply (real boards run 4.2).</summary>
     public byte VersionMinor { get; set; } = 2;
 
+    /// <summary>
+    /// Negate joystick Y in AnalogReply. Only the host sees the flip — local
+    /// state (<see cref="GetAxis"/>, the panel's dot and readout) keeps the
+    /// physical stick direction.
+    /// </summary>
+    public bool InvertJoystickY { get; set; }
+
     /// <summary>
     /// When true, retry exhaustion leaves the analog reply path wedged (the
     /// v4.2 latch-leak bug) until a host ResetRequest clears it.
@@ -274,6 +281,8 @@ public sealed class VRioDevice
                     y = _axes[(int)RioAxis.JoystickY];
                     x = _axes[(int)RioAxis.JoystickX];
                 }
+                if (InvertJoystickY) // clamp: -Min (8192) is one past the 14-bit Max
+                    y = (short)Math.Min(AnalogCodec.Max, -y);
                 Send(PacketBuilder.AnalogReply(t, l, r, y, x));
                 break;