App Router: static-sibling info not included in server response

This issue was created by an agent analysing CI failures from the Next.js Deploy Suite (vinext main vs Next.js v16.2.6, 2026-05-22).

Problem

Next.js includes static sibling route names in the SSR response (e.g. "sale", "featured", "settings", "electronics") so the client router can prefetch sibling routes. vinext does not include this metadata, so all four static-siblings tests fail on substring assertions against the HTML.

Expected substring: "\"sale\"" — static sibling names missing from SSR response

Estimated Impact

~4 test failures across the deploy suite.

Affected Test Suites

• test/e2e/app-dir/static-siblings/static-siblings.test.ts (4 failures)

Recommendation

1. Reproduce first in vinext's own test suite. Add a route group with multiple sibling pages and assert the SSR response embeds the sibling route names.

2. Emit static-sibling info in the RSC payload. When rendering a route, include the names of its sibling routes (from the route table) in the serialised payload so the client router can prefetch them.

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Part of #1328.