Pages Router: deployment-id header missing on SSG `_next/data` routes

[james-elicx]

This issue was created by an agent auditing CI failures from the Next.js Deploy Suite (vinext main vs Next.js v16.2.6, 2026-06-08).

Problem

The deployment-id header is not emitted on Pages Router SSG data routes, so deployment-skew hard-navigation cannot be triggered for _next/data responses.

Affected Test Suites

• test/e2e/pages-ssg-data-deployment-skew/pages-ssg-data-deployment-skew.test.ts (1 failure)
  • hard navigate when a new deployment occurs > header with deployment id > header is set on data routes

Estimated impact

~1 e2e failure.

Notes

The App Router segment-cache variant (segment-cache/deployment-skew) is tracked under #1335; this is the Pages-Router _next/data path.

Recommendation

1. Reproduce: request a Pages Router _next/data/.../page.json route and assert the deployment-id response header is present.
2. Set the deployment-id header on the Pages data-route responses so the client can detect skew and hard-navigate.