Client layer: spread requests across a pool of endpoints.

Picks an endpoint from the pool named in the opts, rewrites the request to target it, and records the outcome so a failing endpoint is ejected and a recovering one is probed back in. Selection load and health live in livery_client_balance_store. Add it with livery_client:balance/1.

Opts: name (required), endpoints (a list of base URLs, or a {Module, Arg} discovery pair), policy (p2c default | round_robin), eject_after (consecutive failures to eject, default 5), eject_for (ms ejected before a half-open probe, default 10000), fail_status (what counts as a failure: a fun((status()) -> boolean()) or a list of statuses; default treats any status >= 500 and any {error, _} as a failure).