Warm standby: follower must keep reading the stream while it reconciles a large graph #356

From the EN-12 part 1 review (#283): the follower reconciles a whole graph in spawn_blocking without reading the socket; the primary's send times out after max(timeout_ms, 10 s) and drops the connection, and the follower restarts the resync from scratch. Decouple socket reads from reconcile (bounded buffer with backpressure), so large graphs never cause a resync loop. Acceptance: resync of a graph 10x the measured 1M-fact case under steady commits completes on the first attempt.