(7/9) Prefetch and dedupe export fallback data requests

[feedthejim]

Context

After hard loads are correct, the next step is making client navigation to unknown params efficient instead of repeatedly rediscovering the same fallback artifact.

What This PR Does

• prefetches fallback data for unknown-param links before navigation
• dedupes direct fallback fetches and critical-path fallback response reuse
• adds the fetch-server-response and fixture coverage needed to prove those paths

Why This Is Separate

This PR is about the pre-hydration navigation path only. The next PR handles what happens after the app has hydrated and can remember more state.

Not In This PR

• hydrated router-state reuse
• unmatched-route recovery with app not-found

Validation

• pnpm --filter=next build
• pnpm testonly packages/next/src/client/components/router-reducer/fetch-server-response.test.ts packages/next/src/client/output-export-fallback.test.ts
• NEXT_SKIP_ISOLATE=1 NEXT_TEST_MODE=start pnpm testheadless test/e2e/app-dir-export/test/dynamic-fallback-cache-components.test.ts

Stack

Part 7 of 9 in the output-export fallback stack.

1. #93032 (1/9) Add the basic runtime for app-router fallbacks in output: 'export'``
2. #93025 (2/9) Cover output: 'export' fallbacks across app-router route shapes
3. #93026 (3/9) Prefer the most specific route for overlapping export fallbacks
4. #93033 (4/9) Reject request-only APIs in output: 'export' fallback pages
5. #93027 (5/9) Define sync IO rules for output: 'export' fallback pages
6. #93028 (6/9) Hide the export fallback bootstrap document on hard loads
7. #93029 (7/9) Prefetch and dedupe export fallback data requests (this PR)
8. #93030 (8/9) Reuse learned export fallback paths after hydration
9. #93031 (9/9) Use app not-found when no export fallback route matches

[github-actions]

All broken links are now fixed, thank you!