Shared CSS chunks vanish on client-side navigation after #7157 #7172

After #7157, in production builds, navigating client-side between two
routes that share a CSS chunk removes that shared <link rel="stylesheet">
from <head> and never restores it. The destination route renders unstyled.

Reproduction

https://github.com/ZacharyL2/tsr-unmatched-assets-repro

git clone https://github.com/ZacharyL2/tsr-unmatched-assets-repro
cd tsr-unmatched-assets-repro
# remove pnpm.patchedDependencies in package.json to see the bug
pnpm install && pnpm build && pnpm start

Open /a, click the /b link, see the shared <Card> lose its style.

Cause

Asset.tsx renders <link rel="stylesheet"> without a precedence prop,
so React 19 doesn't treat it as a hoistable resource. On client-side nav,
React unmounts the previous route's <link> during reconciliation, and
Vite's preload helper (which has an in-memory dep cache) refuses to
re-insert it because it considers the module already loaded.

#7157's includeUnmatchedRouteAssets: false is what makes this visible:
the dehydrated manifest no longer carries the destination route's assets,
so useTags() has nothing to render after navigation.

Full diagnosis (3 layers, with file:line refs) in the repro README.

Proposed fix

// packages/react-router/src/Asset.tsx
   case 'link':
-    return <link {...attrs} nonce={nonce} suppressHydrationWarning />
+    return (
+      <link
+        {...attrs}
+        precedence={
+          attrs?.precedence ??
+          (attrs?.rel === 'stylesheet' ? 'default' : undefined)
+        }
+        nonce={nonce}
+        suppressHydrationWarning
+      />
+    )

Verified in the repro: this single change fully fixes the bug while
keeping #7157's payload optimization. User-set precedence is respected;
non-stylesheet links unaffected.