fix: wait for async loaders before executing head functions

The bug: When loaders run asynchronously (stale-while-revalidate),
loadRouteMatch returns immediately while the loader runs in the background.
Promise.allSettled(inner.matchPromises) waits for loadRouteMatch promises,
not the actual loader completion, causing head() to execute with undefined
loaderData.

The fix: After loadRouteMatch promises settle, explicitly wait for all
loaderPromises to complete before executing head functions. This ensures
loaderData is available when head() executes.

Reproduction scenario:
1. Navigate to authenticated route (e.g., /article/123)
2. Delete auth cookie, reload (shows 'not found')
3. Login, redirect to dashboard
4. Click back button to /article/123
   - Before fix: Article loads but title shows fallback (loaderData undefined)
   - After fix: Article loads with correct title (loaderData available)

Fixes the issue identified in PR #6093 follow-up investigation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: yanghuidong

packages/router-core/src/load-matches.ts

@@ -900,6 +900,16 @@ export async function loadMatches(arg: {
     // Use allSettled to ensure all loaders complete regardless of success/failure
     const results = await Promise.allSettled(inner.matchPromises)
 
+    // Wait for async loaders to complete before executing head functions
+    // loadRouteMatch may return immediately while loaders run asynchronously in the background
+    // We need to wait for the actual loaderPromise, not just the loadRouteMatch promise
+    await Promise.all(
+      inner.matches.map((match) => {
+        const currentMatch = inner.router.getMatch(match.id)
+        return currentMatch?._nonReactive.loaderPromise || Promise.resolve()
+      }),
+    )
+
     const failures = results
       // TODO when we drop support for TS 5.4, we can use the built-in type guard for PromiseRejectedResult
       .filter(