Persist browser sessions without delaying hydration
Let hydration and ordinary events run while the socket-delivered session credential is exchanged for an HttpOnly cookie. Credentials are never written to browser storage; routing-token and freshness keys are scoped to the backend URL. Web Locks coordinate simultaneous cold tabs, and duplicate tabs adopt server-issued replacement tokens.
Idle timers and visibility restoration request fresh credentials over the authenticated socket. Failed HTTP exchanges retry with bounded backoff while readiness stays pending. Success releases waiting uploads and Enterprise HTTP helpers; teardown aborts the exchange and rejects readiness. When an existing cookie replaces the provisional token, readiness waits until the replacement socket connects.
Uploads reserve their cancellation controller before waiting, so cancellation works immediately. Waiting uploads do not block ordinary WebSocket events. Transport warmup for established sessions and CONNECT-packet hydration remain intact.
Depends on #7362; real browser and deployment coverage is in #7364. Enterprise helper compatibility is in reflex-dev/reflex-enterprise#240. Rebased on main d02ba5409.
Validation: all 13 frontend runtime/lifecycle wrapper tests pass, including the actual upload helper, idle renewal, retry recovery, cancellation, late responses after teardown, and cookie-replacement reconnects. The new failure/reconnect regressions fail before their fixes. Full assembled stack: 12,147 unit tests passed with 80.15% coverage.
Related to ENG-12919, ENG-9088.