fix(interception): address codex review issues in sibling intercept implementation

- Pass effectiveParams to resolveAppPageHead so generateMetadata/generateViewport
  sees the intercepted route's params instead of the source route's params
- Rename SIBLING_INTERCEPT_SLOT_NAME from __page to __vinext_sibling_intercept
  to prevent collision with a user @__page parallel route directory
- Restore middleware-rewrite fixture link without locale prefix so the
  interception-dynamic-segment-middleware spec exercises the intended
  middleware rewrite path

packages/vinext/src/routing/app-route-graph.ts

@@ -367,7 +367,7 @@ function createAppRouteGraphDefaultId(slotId: string): string {
   return `default:${slotId}`;
 }
 
-const SIBLING_INTERCEPT_SLOT_NAME = "__page";
+const SIBLING_INTERCEPT_SLOT_NAME = "__vinext_sibling_intercept";
 function createAppRouteGraphSiblingInterceptSlotId(sourcePattern: string): string {
   return createAppRouteGraphSlotId(SIBLING_INTERCEPT_SLOT_NAME, sourcePattern);
 }

packages/vinext/src/server/app-page-element-builder.ts

@@ -206,7 +206,7 @@ export async function buildPageElements<
       routeSegments: route.routeSegments ?? [],
       slots: route.slots ?? null,
     }),
-    params,
+    params: effectiveParams,
     routePath: route.pattern,
     routeSegments: route.routeSegments ?? null,
     searchParams,

tests/fixtures/app-basic/app/interception-mw/[locale]/page.tsx

@@ -4,7 +4,9 @@ export default async function Page({ params }: { params: Promise<{ locale: strin
   const { locale } = await params;
   return (
     <div>
-      <Link href={`/interception-mw/${locale}/foo/p/1`} id="link-foo-p-1">
+      {/* Link without locale — middleware rewrites /interception-mw/foo/p/1
+          to /interception-mw/en/foo/p/1 so interception fires */}
+      <Link href="/interception-mw/foo/p/1" id="link-foo-p-1">
         Foo
       </Link>
     </div>