What?

Convert FsOutput into a string-discriminated union so each filesystem output exposes only the fields valid for its type.

This PR is stacked on #94157.

Why?

The previous optional-field shape allowed invalid combinations and required downstream optional checks even when a route or filesystem path must exist. Narrower variants make those invariants explicit and expose invalid construction paths during type checking.

How?

• Define distinct page, app, static-file, image, and development virtual output variants.
• Require route metadata for page and app outputs and filesystem serving fields for static outputs.
• Use a separate page/app ensure input carrying the request path.
• Narrow outputs by type in route resolution and router dispatch before accessing specialized fields.
• Forward image requests explicitly and assert that development virtual items are consumed by the development bundler.

Verification

• pnpm --filter=next types
• pnpm --filter=next build
• NEXT_TEST_PREFER_OFFLINE=1 pnpm test-dev-turbo test/e2e/dynamic-routing/dynamic-routing.test.ts test/e2e/middleware-general/test/index.test.ts -t "should resolve dynamic route href for page added later|should only contain middleware route in dev middleware manifest"
• NEXT_TEST_PREFER_OFFLINE=1 pnpm test-dev-webpack test/e2e/dynamic-routing/dynamic-routing.test.ts test/e2e/middleware-general/test/index.test.ts -t "should resolve dynamic route href for page added later|should only contain middleware route in dev middleware manifest"
• Manually verified App Router, Pages Router, public files, _next/static, image optimization, static metadata, development virtual manifests, and 404 handling under Turbopack and webpack.
• npx eslint --config eslint.config.mjs packages/next/src/server/lib/router-server.ts packages/next/src/server/lib/router-utils/filesystem.ts packages/next/src/server/lib/router-utils/resolve-routes.ts packages/next/src/server/lib/router-utils/setup-dev-bundler.ts
• git diff --check