Strip story marker characters from placed plain text files

Found while testing crates/textimport/src/lib.rs:140 (plain) and crates/engine/src/cmd/place_text.rs:372 (place_text)

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/links.rs

@@ -1092,6 +1092,16 @@ mod text_link_tests {
         let _ = std::fs::remove_dir_all(&dir);
     }
 
+    #[test]
+    fn placed_plain_text_drops_story_markers() {
+        let mut s = Session::new();
+        s.execute("file.new", &json!({})).unwrap();
+        let b64 = crate::cmd::base64_encode("a\u{E009}b\u{E000}c\u{E002}d".as_bytes());
+        let r = s.execute("file.place", &json!({"base64": b64, "name": "n.txt", "rect": [72, 72, 300, 200]})).unwrap();
+        let st = s.doc().unwrap().doc.story(StoryId(r["story"].as_u64().unwrap())).unwrap();
+        assert_eq!(st.text, "abcd");
+    }
+
     #[test]
     fn create_links_when_placing_text_files_preference() {
         let dir = std::env::temp_dir().join(format!("dc-text-link-pref-{}", std::process::id()));

crates/textimport/src/lib.rs

@@ -117,9 +117,12 @@ pub fn import(name: &str, bytes: &[u8]) -> Result<Imported, ImportError> {
     Ok(plain(&tagged::decode(bytes)))
 }
 
-/// Plain text: line breaks become paragraphs (CRLF / CR normalised).
+/// Plain text: line breaks become paragraphs (CRLF / CR normalised). The private-use characters
+/// (U+E000–U+E1FF) that stories use as markers are dropped, since a stray one would be read as a
+/// marker without its payload.
 pub fn plain(text: &str) -> Imported {
     let t = text.trim_start_matches('\u{FEFF}').replace("\r\n", "\n").replace('\r', "\n");
+    let t: String = t.chars().filter(|c| !('\u{E000}'..='\u{E1FF}').contains(c)).collect();
     let story = Story::with_text(designcraft_doc::StoryId(0), &t, Default::default());
     Imported { story, para_styles: vec![], char_styles: vec![], warnings: vec![] }
 }
@@ -137,6 +140,12 @@ mod tests {
         assert!(matches!(import("x.doc", b"\xD0\xCF"), Err(ImportError::Unsupported(_))));
     }
 
+    #[test]
+    fn plain_text_drops_story_markers() {
+        let i = import("a.txt", "a\u{E009}b\u{E000}c\u{E002}d".as_bytes()).unwrap();
+        assert_eq!(i.story.text, "abcd");
+    }
+
     #[test]
     fn plain_text_utf16() {
         let mut le = vec![0xFF, 0xFE];