Type: thread checks look up the threads once per command

Dynamic Text, Dynamic Text Presets, Center on Top and Center on
Bottom, and Area Type Options with Block on checked each object for a
thread with its own search of the document's threads. A selection of
many objects in a document with many threads repeated that search for
each object, in the commands' enabled checks as well as when they ran.

The checks now build one lookup from frame to thread for the whole
selection. An object counts as threaded by the first thread that lists
it, as before, so the commands refuse the same frames, list them in the
same order and leave the document and the undo history as they were.

crates/engine/src/cmd/threads/tests.rs

@@ -245,6 +245,28 @@ fn a_frame_left_alone_by_a_dropped_repeat_stops_continuing() {
     assert_eq!(t.plain_text(), "Tail");
 }
 
+/// [`ThreadIndex::threaded`] answers as [`story_of`] does, by the first thread that lists an
+/// object, also for threads listed more than once, with point type, other art and missing ids.
+#[test]
+fn thread_index_matches_story_of() {
+    let (mut s, a, b) = setup();
+    s.execute("text.thread.create", &json!({})).unwrap();
+    let c = s.execute("text.create", &json!({"x": 400, "y": 10, "text": "C", "area": {"width": 150, "height": 60}})).unwrap()["id"].as_u64().unwrap();
+    let p = s.execute("text.create", &json!({"x": 10, "y": 250, "text": "Point"})).unwrap()["id"].as_u64().unwrap();
+    let r = s.execute("shape.rectangle", &json!({"x": 400, "y": 200, "width": 50, "height": 50})).unwrap()["id"].as_u64().unwrap();
+    let mut d = (*s.doc().unwrap().doc).clone();
+    let [fa, fb, fc, fp, fr, fx] = [a, b, c, p, r, 9_999_999].map(NodeId);
+    // a's first thread has one frame beside point type, c's has one beside a rectangle, and b's
+    // lists a twice.
+    d.text_threads = vec![vec![fp, fa], vec![fa, fb, fa], vec![fc, fr], vec![fc, fb], vec![fx, fb]];
+    let index = ThreadIndex::of(&d);
+    let got: Vec<(bool, bool)> = [fa, fb, fc, fp, fr, fx].iter().map(|id| (index.threaded(*id), !story_of(&d, *id).is_empty())).collect();
+    let want: Vec<(bool, bool)> = [false, true, false, false, false, false].iter().map(|t| (*t, *t)).collect();
+    assert_eq!(got, want);
+    d.text_threads.clear();
+    assert!(!ThreadIndex::of(&d).threaded(fb));
+}
+
 #[test]
 fn edits_copy_only_the_stories_of_threads_whose_frames_changed() {
     let (mut s, a, b) = setup();