Durable consumer

on every application frame:
    verify the active session and contiguous frameSequence
on new hello:
    create a local session epoch and fence old callbacks
    validate protocol, scope and history; sequence starts at 1
    authenticate; subscribe with saved token or explicit first live_cut
on subscribed:
    validate binding and cut
    start bounded staging for this session and generation
    retain sourceQuality snapshot with its cut
on batch:
    validate counts, ordinals and supported payloads
    stage events in journal order, not observedAt order
on checkpoint:
    validate active session, generation and covered batches
    BEGIN local durable transaction
        for every staged event through this checkpoint:
            identical saved eventId: do not repeat its effect
            conflicting saved eventId: abort processing
            otherwise: apply entity/revision/correction and save dedup
        save quality state and subscription binding
        at the cut: reconcile replay quality with the cut snapshot
        save checkpoint position and its entire opaque token
    COMMIT
    clear accepted staging
    if session is still active: ACK its connection and generation
on caught_up:
    mark the fixed cut reached; do not change transaction finality
on transport failure:
    discard uncommitted staging; retain committed state
    reconnect with finite backoff and the saved durable token
on reset_required:
    stop automatic continuation; obtain explicit gap/start decision