Fix #2907: say that text animator indexes count from 1

layer.addTextAnimatorProperty, layer.addTextSelector and
text.animatorFontAxes take `animator: uid|index`, counted from 1 as the
code and `#n` references do, but the parameters didn't say so and
`animator: 0` failed with only "no animator 0". The parameters now say
1-based, and the error says what the value can be.

crates/engine/src/cmd/text_anim.rs

@@ -59,7 +59,11 @@ fn animator_uid(s: &Session, l: &Layer, p: &Value, cmd: &str) -> Result<u64> {
         if let Some(g) = list.iter().find(|g| g.uid == n) {
             return Ok(g.uid);
         }
-        return list.get((n as usize).wrapping_sub(1)).map(|g| g.uid).ok_or_else(|| bad(cmd, format!("no animator {n}")));
+        let count = if list.len() == 1 { "1 animator".to_string() } else { format!("{} animators", list.len()) };
+        return list
+            .get((n as usize).wrapping_sub(1))
+            .map(|g| g.uid)
+            .ok_or_else(|| bad(cmd, format!("no animator {n}: `animator` is a uid or a 1-based index, and the layer has {count}")));
     }
     for (lid, uid) in &s.state.selected_props {
         if *lid != l.id {
@@ -454,7 +458,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Add Property",
             [],
             None,
-            "{layer?, animator?: uid|index, property: (see layer.addTextAnimator)}",
+            "{layer?, animator?: uid|index (1-based), property: (see layer.addTextAnimator)}",
             has_animator,
             add_animator_prop
         ),
@@ -463,7 +467,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Add Text Selector",
             [],
             None,
-            "{layer?, animator?: uid|index, kind: range|wiggly|expression}",
+            "{layer?, animator?: uid|index (1-based), kind: range|wiggly|expression}",
             has_animator,
             add_selector
         ),
@@ -482,7 +486,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Variable Font Axes",
             ["Animation", "Animate Text"],
             None,
-            "{layer?, axis?: tag|name (omit to list the font's axes), animator?: uid|index}",
+            "{layer?, axis?: tag|name (omit to list the font's axes), animator?: uid|index (1-based)}",
             has_text_layer,
             font_axes
         ),

crates/engine/src/tests/text.rs

@@ -49,6 +49,27 @@ fn animate_text_adds_animators_with_companion_properties() {
     let _ = s.render(cid, s.time(), Default::default());
 }
 
+/// `animator` counts from 1, as `#n` references do: the commands that take it say so, and 0 fails
+/// with an error that says so (agents reading `uid|index` tried 0).
+#[test]
+fn animator_indices_count_from_1_and_say_so() {
+    let (mut s, t) = session();
+    let uid = s.execute("layer.addTextAnimator", json!({"layer": t, "property": "position"})).unwrap()["animator"].as_u64().unwrap();
+    for id in ["layer.addTextAnimatorProperty", "layer.addTextSelector", "text.animatorFontAxes"] {
+        let spec = crate::cmd::command_specs().iter().find(|c| c.id == id).unwrap();
+        assert!(spec.params.contains("animator?: uid|index (1-based)"), "{id}: {}", spec.params);
+    }
+    for (id, p) in [
+        ("layer.addTextAnimatorProperty", json!({"layer": t, "animator": 0, "property": "fillHue"})),
+        ("layer.addTextSelector", json!({"layer": t, "animator": 0, "kind": "wiggly"})),
+    ] {
+        let e = s.execute(id, p).unwrap_err().to_string();
+        assert!(e.contains("no animator 0") && e.contains("1-based"), "{id}: {e}");
+    }
+    let r = s.execute("layer.addTextSelector", json!({"layer": t, "animator": 1, "kind": "wiggly"})).unwrap();
+    assert_eq!(r["animator"].as_u64(), Some(uid));
+}
+
 #[test]
 fn add_property_and_selectors_with_undo_redo() {
     let (mut s, t) = session();