The router keeps each endpoint's time to first token, smoothed over its last
streamed answers, and the chat profile scores by it beside session, queue,
KV and inflight: the fastest endpoint 1, one twice as slow 0.5, one not yet
measured 1 so a new replica is tried. Load still sees a slow drain from the
engine's own counters; this sees what answers actually took. BACKLOG L4.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>