perf(engine): index variables once when following a library's alias chains (#1968)

* perf(engine): index variables once when following a library's alias chains

* address review: return before indexing variables for an empty seed set

crates/engine/src/assets/library.rs

@@ -277,8 +277,16 @@ pub fn default_selection(doc: &Document, prev: Option<&LibraryManifest>) -> Sele
 fn alias_closure(vars: &Variables, seeds: impl IntoIterator<Item = VariableId>) -> BTreeSet<VariableId> {
     let mut out = BTreeSet::new();
     let mut stack: Vec<VariableId> = seeds.into_iter().collect();
+    if stack.is_empty() {
+        return out;
+    }
+    // Indexed once (the first of equal ids, as `get_by_id` finds it): a scan per step is quadratic on a long alias chain.
+    let mut by_id = std::collections::HashMap::new();
+    for v in &vars.variables {
+        by_id.entry(v.id.as_str()).or_insert(v);
+    }
     while let Some(cur) = stack.pop() {
-        let Some(v) = vars.get_by_id(&cur) else { continue };
+        let Some(v) = by_id.get(cur.as_str()).copied() else { continue };
         if out.len() >= MAX_ASSETS || !out.insert(v.id.clone()) {
             continue;
         }