U4: Implement outbound provider sessions and replay

[blackforestboi]

Implements U4 of the implementation plan.

Depends on U1-U3. Implement exact-origin WSS sessions, header grants, negotiation, cursor sync, claim/lease, pagination, reconnect, backpressure, quotas, rotation, and isolation.

Requirements: R3, R4, R8-R10, R12, R13, R15. Exit: deterministic and soak tests show no lost record, credential leak, starvation, or unbounded growth.