Complete Phase 4 health-gated service discovery

[kacy]

Summary

• gate runtime endpoint eligibility on generation-aware health readiness instead of durable admin-state churn
• replace the fixed, sequential health checker with a bounded worker-based scheduler that rejects stale probe completions
• expose health-checker capacity, queue, and stale-result telemetry in service rollout status, metrics, and service endpoint detail

Commits

• fabc97d Gate endpoint eligibility by health generation
• 5155a7c Add bounded generation-aware health checking

Verification

• zig fmt on touched files
• bounded zig build test runs for:
  • state machine — healthy to unhealthy after retries
  • route handles /v1/status?mode=service_rollout GET
  • handleMetricsPrometheus exposes service rollout metrics
• each timed out after 45 seconds with no emitted failures

Notes

• probe transitions no longer rewrite durable endpoint admin state
• readiness-gated endpoints stay ineligible until a matching-generation probe succeeds
• stale probe completions are dropped by generation and registration epoch before they can republish service membership