Add best-effort HTTP route mirroring

[kacy]

Summary

• add mirror_service as a best-effort shadow target for HTTP routes
• surface primary and mirror route traffic separately in JSON and Prometheus
• mirror HTTP/1.1 and routed HTTP/2 traffic without affecting the primary response path

Validation

• YOQ_SKIP_SLOW_TESTS=1 zig build test
• 1753 passed; 42 skipped; 0 failed