fix(router): scroll to top on cache-miss popstate

Companion to the previous commit. Setting history.scrollRestoration
= 'manual' takes the browser out of the scroll-restoration game on
back/forward — which is great when we have a cached snapshot (we
restore exactly where the user left off) but leaves the cache-miss
case unhandled. Without an explicit scrollTo, the cache-miss
popstate would leave scroll wherever the user was on the page they
popped FROM, which is jarring.

Adds an explicit window.scrollTo(0, 0) in the cache-miss branch of
performNavigation's popstate path — fetchAndApply itself skips its
scroll block when recordHistory=false (which is always the case for
popstate), so this is the only place to put it.

This matches what browser-native scroll restoration would do for a
history entry it has no saved scroll for. The cached-hit branch is
unchanged.

Author: vivek7405

packages/core/src/router-client.js

@@ -609,6 +609,13 @@ async function performNavigation(href, isPopState, frameId) {
           return;
         }
       }
+      // Cache-miss popstate. Browser-native scroll restoration is
+      // disabled (we set scrollRestoration='manual') — so without
+      // explicit handling, scroll would just stay where the user was
+      // on the page they popped FROM. Scroll to top as the reasonable
+      // default; fetchAndApply skips its own scroll handling when
+      // recordHistory=false (which is the case here).
+      if (typeof window !== 'undefined') window.scrollTo(0, 0);
     }
 
     await fetchAndApply(href, frameId, !isPopState, optimisticState, 'GET', null, signal, myToken);