fix: DefaultCleanMatcher.Match incorrectly returns null for same-name files in different directories (#170)

* Initial plan

* fix: match by RelativePath in DefaultCleanMatcher to handle same-name files in different dirs

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* style: replace en dashes with hyphens in test comments

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Co-authored-by: JusterZhu <11714536+JusterZhu@users.noreply.github.com>

* fix: use null-safe string.Equals and remove redundant RelativePath guard in DefaultCleanMatcher.Match

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---------

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Co-authored-by: JusterZhu <11714536+JusterZhu@users.noreply.github.com>

Author: Copilot

src/c#/DifferentialTest/Matchers/MatcherTests.cs

@@ -182,6 +182,36 @@ public void DefaultCleanMatcher_ReturnsNull_WhenNewFileDoesNotExist()
             Assert.Null(result);
         }
 
+        [Fact]
+        public void DefaultCleanMatcher_ReturnsCorrectNode_WhenSameNameFilesExistInDifferentDirectories()
+        {
+            // Arrange - two old files with the same name but different relative paths
+            var fileInRoot = Path.Combine(_testDirectory, "a.txt");
+            var subDir = Path.Combine(_testDirectory, "sub");
+            Directory.CreateDirectory(subDir);
+            var fileInSub = Path.Combine(subDir, "a.txt");
+            File.WriteAllText(fileInRoot, "root");
+            File.WriteAllText(fileInSub, "sub");
+
+            // The new file lives in the sub-directory
+            var newFile = new FileNode { Name = "a.txt", FullName = fileInSub, RelativePath = "sub/a.txt" };
+            var oldFileRoot = new FileNode { Name = "a.txt", FullName = fileInRoot, RelativePath = "a.txt" };
+            var oldFileSub  = new FileNode { Name = "a.txt", FullName = fileInSub,  RelativePath = "sub/a.txt" };
+
+            // oldFileRoot appears first in the list - before the fix FirstOrDefault would
+            // return oldFileRoot, then the RelativePath guard would return null, hiding oldFileSub.
+            var leftNodes = new List<FileNode> { oldFileRoot, oldFileSub };
+
+            var matcher = new DefaultCleanMatcher();
+
+            // Act
+            var result = matcher.Match(newFile, leftNodes);
+
+            // Assert - must find oldFileSub, not be tricked by oldFileRoot
+            Assert.NotNull(result);
+            Assert.Same(oldFileSub, result);
+        }
+
         [Fact]
         public void DefaultCleanMatcher_ReturnsNull_WhenLeftNodesIsEmpty()
         {

src/c#/GeneralUpdate.Differential/Matchers/DefaultCleanMatcher.cs

@@ -30,11 +30,12 @@ public ComparisonResult Compare(string sourcePath, string targetPath)
         /// <inheritdoc/>
         public FileNode? Match(FileNode newFile, IEnumerable<FileNode> leftNodes)
         {
-            var oldFile = leftNodes.FirstOrDefault(i => i.Name.Equals(newFile.Name));
+            var oldFile = leftNodes.FirstOrDefault(i =>
+                string.Equals(i.Name, newFile.Name) &&
+                string.Equals(i.RelativePath, newFile.RelativePath));
             if (oldFile is null) return null;
             if (!File.Exists(oldFile.FullName)) return null;
             if (!File.Exists(newFile.FullName)) return null;
-            if (!string.Equals(oldFile.RelativePath, newFile.RelativePath)) return null;
             return oldFile;
         }
     }