(12/19) Fallback to exported RSC for cache misses

[feedthejim]

Stack position

Part 12 of 19 in the output export dynamic fallback stack.

Previous PR: #93562 (11/19)
Next PR: #93571 (13/19)

Context

The stack is split so build-time output can land behind experimental.outputExportDynamicFallbacks before the client router behavior is enabled. The target behavior is for output: 'export' apps with parameterized App Router routes to emit fallback HTML/RSC artifacts that a later client can resolve for hard loads and soft navigations.

This PR scope: Segment-cache recovery when no route cache entry exists.

A soft navigation can miss the route cache for an unknown param. This PR lets the segment cache fetch the full fallback RSC payload, validate it, and seed reusable cache entries.

What changed

• Adds output-export fallback recovery for route cache misses.
• Writes fallback-derived route and segment data into the cache using the fallback base path.
• Keeps the recovery branch feature-gated so normal export behavior is unchanged when disabled.

Deliberately not included

• Does not add route-shape mismatch fallback-to-not-found yet.
• Does not add manifest conflict resolution.
• Does not change the generic prefetch fetch helper.

Verification

After restacking and autosquashing, I verified the full stack with:

• pnpm --filter=next build
• pnpm testonly packages/next/src/lib/output-export-dynamic-fallback.test.ts packages/next/src/lib/output-export-fallback-routes.test.ts packages/next/src/export/helpers/output-export-fallback.test.ts packages/next/src/server/request/fallback-params.test.ts
• pnpm testonly packages/next/src/client/output-export-fallback.test.ts packages/next/src/client/components/router-reducer/fetch-server-response.test.ts packages/next/src/client/components/router-reducer/create-initial-router-state.test.ts packages/next/src/client/components/segment-cache/cache.test.ts packages/next/src/client/components/segment-cache/vary-path.test.ts
• pnpm testonly packages/next/src/client/components/router-reducer/compute-changed-path.test.ts packages/next/src/client/flight-data-helpers.test.ts packages/next/src/server/app-render/postponed-state.test.ts
• pnpm test-start-turbo test/e2e/app-dir-export/test/dynamic-fallback-cache-components.test.ts test/e2e/app-dir-export/test/dynamic-fallback-route-shapes-cache-components.test.ts test/e2e/app-dir-export/test/dynamic-fallback-cache-components-basepath.test.ts test/e2e/app-dir-export/test/dynamic-fallback-optimizations-cache-components.test.ts test/e2e/app-dir-export/test/dynamic-fallback-known-params-cache-components.test.ts test/e2e/app-dir-export/test/dynamic-fallback-conflict.test.ts
• pnpm test-start-turbo test/e2e/app-dir/output-export-dynamic-fallbacks/output-export-dynamic-fallbacks.test.ts test/e2e/app-dir-export/test/output-export-server-io.test.ts test/e2e/app-dir-export/test/output-export-server-io-use-cache.test.ts test/e2e/app-dir-export/test/dynamic-fallback-server-params.test.ts

[github-actions]

Tests Passed

Commit: fb8e4f1