fix: FASS2 状态帧校验 XOR

错误校验和的 100B 上报不再被当成合法状态解析。

Co-authored-by: Cursor <cursoragent@cursor.com>

StandardScene.Magnetic.Tests/Protocol/Fass2ProtocolGoldenTests.cs

@@ -124,6 +124,24 @@ namespace StandardScene.Magnetic.Tests.Protocol
             Assert.Equal((ushort)500, report.Node.Distance);
         }
 
+        [Fact]
+        public void TryExtractStateFrame_RejectsBadXor()
+        {
+            var frame = Fass2TestFrameBuilder.BuildStateFrame(5, 2, 7, 42);
+            frame[98] ^= 0xFF;
+
+            Assert.False(Fass2Protocol.TryExtractStateFrame(frame, frame.Length, out _));
+        }
+
+        [Fact]
+        public void ParseState_RejectsBadXor()
+        {
+            var frame = Fass2TestFrameBuilder.BuildStateFrame(5, 2, 7, 42);
+            frame[98] ^= 0xFF;
+
+            Assert.Throws<ArgumentException>(() => Fass2Protocol.ParseState(frame));
+        }
+
         [Fact]
         public void NodeMessage_ToBytes_FromBytes_RoundTrip()
         {

StandardScene.Magnetic/Protocol/Fass2Protocol.cs

@@ -141,19 +141,27 @@ namespace StandardScene.Magnetic.Protocol
                     continue;
                 }
 
+                if (!HasValidStateChecksum(buffer, i))
+                {
+                    continue;
+                }
+
                 frame = new byte[StateFrameLength];
                 Buffer.BlockCopy(buffer, i, frame, 0, StateFrameLength);
                 return true;
             }
 
-            if (length == StateFrameLength && buffer[0] == Begin && buffer[StateFrameLength - 1] == End)
+            return false;
+        }
+
+        public static bool HasValidStateChecksum(byte[] bytes, int offset = 0)
+        {
+            if (bytes == null || offset < 0 || offset + StateFrameLength > bytes.Length)
             {
-                frame = new byte[StateFrameLength];
+                return false;
-                Buffer.BlockCopy(buffer, 0, frame, 0, StateFrameLength);
-                return true;
             }
 
-            return false;
+            return bytes[offset + 98] == Xor(bytes, offset + 1, 97);
         }
 
         public static Fass2StateReport ParseState(byte[] bytes)
@@ -166,6 +174,11 @@ namespace StandardScene.Magnetic.Protocol
             {
                 throw new ArgumentException($"invalid FASS2 state frame: begin=0x{bytes[0]:X2}, end=0x{bytes[99]:X2}");
             }
+            if (!HasValidStateChecksum(bytes))
+            {
+                throw new ArgumentException(
+                    $"invalid FASS2 state xor: got=0x{bytes[98]:X2}, expect=0x{Xor(bytes, 1, 97):X2}");
+            }
 
             var nodeBytes = new byte[25];
             Buffer.BlockCopy(bytes, 53, nodeBytes, 0, 25);