Basket persists across in-app navigation

useBasketUrlSync was treating every searchParams change as a URL→Store
authority event. In practice this meant Back from PackagesStep to the
providers map landed on `/` (no `?compare=...`) and the hook called
setAll([]) — wiping the basket.

Changed the semantics so that when an in-app navigation drops the
`?compare=` param but the store still has items, we re-attach the
store's keys to the new URL rather than clearing the store. Shared
links still hydrate the store on initial mount, and the subscribe
that writes store→URL on basket changes is untouched.

With this, a user can:
- Add a package on Provider A's page.
- Back to the providers map (CompareBar stays, URL still shows
  `?compare=parsons:everyday`).
- Navigate into Provider B's page (URL carries the Parsons item forward).
- Add B's package (URL now `?compare=parsons:everyday,rankins:standard`).
- Hit Compare with 2/3 basket.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/demo/shared/state/useBasketUrlSync.ts

@@ -18,8 +18,14 @@ const deserialise = (raw: string | null): string[] =>
  *
  * Mount once near the router root. URL is the source of truth on initial load
  * (so a shared link restores the basket); after that, store changes write
- * through to the URL and external URL changes (back/forward, manual edits)
- * push back into the store.
+ * through to the URL so the current basket is always shareable.
+ *
+ * In-app navigation from a page that carries `?compare=...` to one that
+ * doesn't (e.g. Back from PackagesStep to the providers map) would drop the
+ * param — to avoid wiping the store, we re-attach the store's keys to the
+ * new URL instead of treating the empty URL as a "clear" signal. External
+ * URL changes that DO carry params still push back into the store (shared
+ * links, manual edits, browser Back after a store write).
  */
 export function useBasketUrlSync(): void {
   const [searchParams, setSearchParams] = useSearchParams();
@@ -37,10 +43,27 @@ export function useBasketUrlSync(): void {
       return;
     }
 
-    if (serialise(urlKeys) !== serialise(storeKeys)) {
-      useComparisonBasket.getState().setAll(urlKeys);
+    if (serialise(urlKeys) === serialise(storeKeys)) return;
+
+    // URL empty + store has items → in-app navigation dropped the param.
+    // Re-attach the store's keys so the basket stays sticky across routes
+    // (and the current URL remains shareable).
+    if (urlKeys.length === 0 && storeKeys.length > 0) {
+      setSearchParams(
+        (current) => {
+          const next = new URLSearchParams(current);
+          next.set(PARAM, serialise(storeKeys));
+          return next;
+        },
+        { replace: true },
+      );
+      return;
     }
-  }, [searchParams]);
+
+    // Otherwise URL is authoritative (shared link, manual edit, browser Back
+    // after a store write) — push it into the store.
+    useComparisonBasket.getState().setAll(urlKeys);
+  }, [searchParams, setSearchParams]);
 
   useEffect(() => {
     return useComparisonBasket.subscribe((state, prev) => {