fix: reset previousPartIdsRef on build clear; clarify first-hydration skip comment

app/(app)/(builder)/builder/page.tsx

@@ -618,6 +618,7 @@ function GunbuilderPageContent() {
       if (ids.length === 0) {
         lastHydrateKeyRef.current = "";
         setParts([]);
+        previousPartIdsRef.current = new Set(); // keep in sync so re-selecting fires the event
         setError(null);
         setLoading(false);
         return;
@@ -670,8 +671,9 @@ function GunbuilderPageContent() {
 
         setParts(hydrated);
 
-        // Track newly selected parts (fires after API returns with real names).
-        // Skip the first hydration — that's restoring a saved build, not a new selection.
+        // Track newly selected parts after API fetch — fires with real name, not a numeric ID.
+        // Skip the first hydration: this covers both localStorage build restoration and ?select=
+        // share-link arrivals. Neither represents a live user interaction in the current session.
         if (isFirstPartsHydrationRef.current) {
           isFirstPartsHydrationRef.current = false;
         } else {