fix(react-router): avoid Suspense above root documents (#8055)

* fix(react-router): avoid suspense above root documents

* test(react-start): cover root document hydration

* test(react-router): remove indirect hydration assertion

* perf(react-router): reduce suspense wrapper bundle size

* test(react-start): isolate root hydration regression

* fix(react-router): preserve root suspense in CSR

* refactor(react-router): simplify root suspense checks

* fix(react-router): preserve functional ssr classification