App Router: `global-error.tsx` receives generic digest message and lacks built-in fallback

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

Two related global-error issues: (1) when a server or client component throws and is caught by global-error.tsx, the error.message passed to the boundary in production is the generic An error occurred in the Server Components render... digest message instead of the actual error message; (2) when global-error.tsx itself throws, Next.js falls back to a built-in last-resort UI (an <h1>). vinext renders nothing — Playwright cannot locate any h1 within the timeout.

Expected: "Global error: client page error"
Received: "Global error: An error occurred in the Server Components render..."

page.waitForSelector('h1') timeout — no built-in fallback UI when global-error.tsx throws

Estimated Impact

~3 test failures across the deploy suite.

Affected Test Suites

• test/e2e/app-dir/global-error/basic/index.test.ts (2 failures)
• test/e2e/app-dir/global-error/error-in-global-error/error-in-global-error.test.ts

Recommendation

1. Reproduce first in vinext's own test suite. Add a client component that throws and a global-error.tsx. Assert the boundary's error.message is the original message in dev and the digest in prod. Then add a global-error.tsx that itself throws and assert the built-in fallback renders.

2. Forward real messages to the client boundary. Even in production, the client error boundary should receive enough information (digest+message in dev, just digest in prod) — but currently the message text is being stripped where it should be passed.

3. Implement a last-resort fallback UI. When global-error.tsx itself throws, render Next.js's built-in <h1> fallback so users do not see a blank page.

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Part of #1328.