Add L7 proxy circuit breaking

[kacy]

Summary

• add circuit-breaker policy primitives for the L7 proxy
• track per-endpoint open and half-open circuit state in proxy runtime and expose it in rollout status and Prometheus
• re-resolve upstreams on retries so a tripped endpoint can be bypassed, and add coverage for circuit-open endpoint selection and retry-to-different-endpoint behavior

Validation

• zig fmt on touched files
• single-process targeted zig build test -Doptimize=ReleaseSafe -- --test-filter ... runs for proxy runtime, proxy forwarding, and rollout metrics
• a logging-format compile bug surfaced during the first targeted build and was fixed before rerunning
• subsequent targeted runs hit the explicit timeout without emitting failures