Adaptive replication routing: dynamic fan-out under egress back-pressure

## Overview

Dynamically adjust replication routing topology based on back-pressure from egress network limits, switching to fan-out only when needed.

## Design

Under normal conditions, each node replicates directly to all subscribers (lowest latency). When egress limits are hit:

1. Back-pressure detected in outgoing replication threads → reported to main thread
2. Main thread identifies which outgoing connections are back-pressured
3. Signals a child thread to suspend subscription for the highest-latency remote node
4. Child thread sends subscription suspension request; keeps connection open
5. Remote node redirects that subscriber to another node (re-subscribes indirectly)
6. Main thread monitors: resumes direct subscriptions when back-pressure clears

## Key constraint

Harper doesn't proactively push replication — nodes subscribe to receive it. Back-pressure detection must bridge the gap between the sending thread (which sees network limits) and the main thread (which manages topology).

🤖 Filed by Claude on behalf of Kris.