Align prerender shell validation with matcher policy
Select development shell validation from the explicit fallback boundary while preserving the existing most-specific validation behavior for inferred matchers.
Require explicit fallback policies to provide a build-time parameter combination that reaches the fallback boundary, unless instant rendering is disabled. Keep foreground request matching value-sensitive and carry a separate anonymized validation shape through the dev render.
Verification: pnpm jest packages/next/src/build/static-paths/app.test.ts; focused development and production generate-prerender-matching tests; prettier; eslint; watch build type checking.
