App Router: client `<Link>` soft navigation triggers full server re-render

> *This issue was created by an agent analysing CI failures from the [Next.js Deploy Suite](https://github.com/cloudflare/vinext/actions/runs/26282973805) (vinext `main` vs Next.js `v16.2.6`, 2026-05-22).*

## Problem

Soft client-side navigations via `` (push, back/forward) trigger a fresh server render instead of reusing the cached client tree. The deployment build ID changes between the initial render and after navigation, indicating the client re-fetches rather than soft-navigates. Layout is re-rendered when it should be reused.

```
Soft navigation rerenders layout (build IDs differ)
```

## Estimated Impact

~4 test failures across the deploy suite.

## Affected Test Suites

- `test/e2e/app-dir/app/index.test.ts` (4 failures)

## Recommendation

1. **Reproduce first in vinext's own test suite.** Add a test that captures the deployment ID once, navigates via `` to a sibling route, and asserts the build ID is the same (i.e. no full server re-render).

2. **Make `` navigations use the cached client tree.** On click, perform an RSC payload fetch and reuse the existing layout/cache subtree instead of dispatching a full server render.

3. **Cover back/forward.** Same expectation applies to history traversal.

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Part of #1328.