fix: insert heading-start newlines before hidden prefixes

Tests/ConjureKitTests/NotesHeadingTests.swift

@@ -49,6 +49,38 @@ func headingsFollowVisibleParagraphEditingAndRoundTrip() throws {
             #expect(editor.markdownString == after)
         }
     }
+    for before in ["\n# Title", "\n## Title", "Text\n\n###### Title", "- [ ] 🦊\n\n## Title"] {
+        reset(before)
+        editor.undoManager?.beginUndoGrouping()
+        editor.doCommand(by: #selector(NSResponder.deleteBackward(_:)))
+        editor.undoManager?.endUndoGrouping()
+        let joined = editor.markdownString
+        editor.breakUndoCoalescing()
+        editor.undoManager?.removeAllActions()
+        editor.undoManager?.beginUndoGrouping()
+        editor.doCommand(by: #selector(NSResponder.insertNewline(_:)))
+        editor.undoManager?.endUndoGrouping()
+        #expect(editor.markdownString == before)
+        #expect(editor.selectedRange() == NSRange(location: (editor.string as NSString).range(of: "Title").location, length: 0))
+        editor.undoManager?.undo()
+        #expect(editor.markdownString == joined)
+        editor.undoManager?.redo()
+        #expect(editor.markdownString == before)
+        #expect(controller.save())
+        #expect(try NotesStore(directory: directory).notes.first?.contents.string == before)
+    }
+    for (before, position, after) in [
+        ("## Title", 3, "\n## Title"),
+        ("## Title", 5, "## Ti\ntle"),
+        ("## Title", 8, "## Title\n"),
+        ("Text\n## Title", 4, "Text\n\n## Title"),
+        ("```text\n## Title\n```", 11, "```text\n## \nTitle\n```")
+    ] {
+        reset(before)
+        editor.setSelectedRange(NSRange(location: position, length: 0))
+        editor.doCommand(by: #selector(NSResponder.insertNewline(_:)))
+        #expect(editor.markdownString == after)
+    }
     for level in 1...6 {
         reset("### Title")
         editor.undoManager?.beginUndoGrouping()