Add Document.title test

GeminiFormatTests/DocumentTests.swift

@@ -53,4 +53,28 @@ test
 """)
     }
     
+    func testGetTitle() {
+        let h1 = Document(url: URL(string: "gemini://example.com/")!, lines: [
+            .heading("Test", level: .h1)
+        ])
+        XCTAssertEqual(h1.title, "Test")
+        
+        let h2 = Document(url: URL(string: "gemini://example.com/")!, lines: [
+            .heading("Test", level: .h2)
+        ])
+        XCTAssertEqual(h2.title, "Test")
+        
+        let h3 = Document(url: URL(string: "gemini://example.com/")!, lines: [
+            .heading("Test", level: .h3)
+        ])
+        XCTAssertEqual(h3.title, "Test")
+        
+        let multiple = Document(url: URL(string: "gemini://example.com/")!, lines: [
+            .heading("Two", level: .h2),
+            .heading("One", level: .h1),
+            .heading("Three", level: .h3),
+        ])
+        XCTAssertEqual(multiple.title, "Two")
+    }
+
 }