feat(ft546): persist shadow comparison evidence

ruoyi-wcs-shadow/src/main/java/com/ruoyi/shadow/evidence/ShadowDifference.java

@@ -0,0 +1,32 @@
+package com.ruoyi.shadow.evidence;
+
+import java.util.Objects;
+
+public final class ShadowDifference {
+    private final ShadowDifferenceCategory category;
+    private final String reasonCode;
+    private final String expectedJson;
+    private final String actualJson;
+
+    public ShadowDifference(ShadowDifferenceCategory category,
+                            String reasonCode,
+                            String expectedJson,
+                            String actualJson) {
+        this.category = Objects.requireNonNull(category, "category");
+        this.reasonCode = required(reasonCode, "reasonCode");
+        this.expectedJson = expectedJson;
+        this.actualJson = actualJson;
+    }
+
+    public ShadowDifferenceCategory getCategory() { return category; }
+    public String getReasonCode() { return reasonCode; }
+    public String getExpectedJson() { return expectedJson; }
+    public String getActualJson() { return actualJson; }
+
+    private static String required(String value, String name) {
+        if (value == null || value.trim().isEmpty()) {
+            throw new IllegalArgumentException(name + " must not be blank");
+        }
+        return value.trim();
+    }
+}