fix(script): keep After Effects Undo command ID 16 stable (#812)

crates/script/src/prelude.js

@@ -1798,8 +1798,13 @@ var app = {
   beginSuppressDialogs: function () {},
   endSuppressDialogs: function () {},
   purge: function () { __call("edit.purge", { what: "all" }); },
-  findMenuCommandId: function (name) { return __get("menuCommandId", { name: String(name) }); },
+  findMenuCommandId: function (name) {
+    // Well-known After Effects ids must not shift when EffectCraft registers a command.
+    if (String(name) === "Undo") return 16;
+    return __get("menuCommandId", { name: String(name) });
+  },
   executeCommand: function (id) {
+    if (id === 16) return __call("edit.undo", {});
     var cmd = typeof id === "number" ? __get("commandAt", { index: id }) : String(id);
     if (!cmd) throw __err("Unknown command id " + id);
     return __call(cmd, {});

crates/script/src/tests.rs

@@ -730,3 +730,20 @@ fn import_options_sequence_picks_one_still_or_the_run() {
     assert_eq!(kind_of(&s, "shot_[0001-0003].png"), FootageKind::Sequence);
     let _ = std::fs::remove_dir_all(&dir);
 }
+
+#[test]
+fn script_undo_uses_stable_after_effects_command_id() {
+    let mut s = session();
+    let lookup = ok(&mut s, "app.findMenuCommandId('Undo')");
+    assert_eq!(lookup.result, json!(16));
+    // ID 16 used to index an unrelated command as the registry grew. It must
+    // undo the preceding layer creation rather than dispatching by position.
+    let r = ok(
+        &mut s,
+        "var c = app.project.items.addComp('Undo ID', 100, 100, 1, 1, 24); c.layers.addNull(); app.executeCommand(16); c.numLayers",
+    );
+    assert_eq!(r.result, json!(0));
+    assert_eq!(s.project.comp(ItemId(1)).unwrap().layers.len(), 0);
+    // Lookup of other command names still uses the existing registry.
+    assert!(ok(&mut s, "app.findMenuCommandId('New Project')").result.as_u64().unwrap_or(0) > 0);
+}