RSC segment cache prefetch protocol not implemented

> *This issue was created by an agent analysing CI failures from the [Next.js Deploy Suite](https://github.com/cloudflare/vinext/actions/runs/26163751905) (vinext `main` vs Next.js `v16.2.6`, 2026-05-20).*

## Problem

vinext does not implement the Next.js 15+ RSC segment cache prefetch protocol. This is a new system where the client-side router prefetches individual route segments as RSC payloads, enabling more granular caching and faster navigations.

Tests wait for segment-level prefetch requests (intercepted via Playwright network monitoring) and expect to receive RSC segment payloads. These requests never fire because vinext has no segment prefetch endpoint.

```
Timed out waiting for a request to be initiated.

Expected: > 0
Received: 0
expect(bodies.length).toBeGreaterThan(0)
```

## Estimated Impact

~66 test failures across the deploy suite.

## Affected Test Suites

- `test/e2e/app-dir/segment-cache/vary-params/vary-params.test.ts` (15 failures)
- `test/e2e/app-dir/segment-cache/vary-params-base-dynamic/vary-params-base-dynamic.test.ts` (15 failures)
- `test/e2e/app-dir/segment-cache/basic/segment-cache-basic.test.ts` (9 failures)
- `test/e2e/app-dir/segment-cache/cached-navigations/cached-navigations.test.ts` (8 failures)
- `test/e2e/app-dir/segment-cache/prefetch-inlining/prefetch-inlining.test.ts` (7 failures)
- `test/e2e/app-dir/app-client-cache/client-cache.original.test.ts` (9 failures)
- `test/e2e/app-dir/app-client-cache/client-cache.experimental.test.ts` (7 failures)
- `test/e2e/app-dir/app-prefetch/prefetching.test.ts` (9 failures)
- `test/e2e/app-dir/app-prefetch-false/*.test.ts`
- `test/e2e/app-dir/app-prefetch-static/*.test.ts`

## Recommendation

1. **Reproduce first in vinext's own test suite.** Add a test that navigates between App Router pages and verifies prefetch requests are made. Confirm it fails.

2. **Study the Next.js segment cache implementation.** This is a large subsystem. Search `.nextjs-ref/packages/next/src/` for segment cache, prefetch, and RSC flight payload serving. Understand the endpoint format, request/response protocol, and how segments are keyed.

3. **This is a significant new feature.** Consider scoping an initial implementation that handles the basic prefetch protocol (serving RSC payloads for individual segments on prefetch requests) before tackling advanced features like segment-level caching, vary-by-params, and memory pressure eviction.

4. **Note:** This may be intentionally deferred given its complexity. Consider whether these tests should be skipped in the interim.