first-backoff tuning: reconnect pays 200ms-2s per tear

open   by a73x

Labels: backlog, perf, client

Carried since M7. A reconnect after a tear can pay a 200ms-2s backoff step; measured ~4x win available on fast links, ~2x on slow (decisions.md M7). The schedule is now a documented inline expression in client.zig's reconnect (0 -> 200 -> x2 capped 2000); M15's survey suggested extracting it as a pure pinned nextBackoff() when touched.