Restore Diff level working-tree change

- Add a dedicated native Diff fixture that commits baseline app.rb content and leaves an unstaged line-26 change from data.json to server.json.
- Update the Diff level setup metadata so app.rb is no longer represented as an empty file.
- Add a native runtime regression for `git diff` showing the expected line-26 hunk.

app/src/test/java/solutions/tretter/githugandroid/GitSandboxEngineTest.kt

@@ -209,6 +209,29 @@ class GitSandboxEngineTest {
         }
     }
 
+    @Test
+    fun nativeDiffLevelShowsLine26Change() {
+        val git = testGitBinary()
+        assumeTrue(git.exists() && git.canExecute())
+        val root = Files.createTempDirectory("githug-diff-level").toFile()
+        try {
+            val runtime = GitRepositoryRuntime(root, git)
+            val level = diffLevel()
+            val repo = runtime.prepareLevel(level)
+
+            val appFile = repo.files.single { it.name == "app.rb" }
+            assertTrue(appFile.content.contains("server.json"))
+
+            val (_, diffOutput) = runtime.execute(level, repo, "git diff")
+
+            assertTrue(diffOutput.any { it.contains("-  @message = get_response('data.json')") })
+            assertTrue(diffOutput.any { it.contains("+  @message = get_response('server.json')") })
+            assertTrue(diffOutput.any { it.contains("@@ -23,7 +23,7 @@") })
+        } finally {
+            root.deleteRecursively()
+        }
+    }
+
     @Test
     fun cdDotDotShortcutMovesToParentDirectory() {
         val repo = RepoState(initialized = true, currentDir = "src/main")