Expose weighted route traffic in JSON status

[kacy]

Summary

• expose aggregate route traffic and per-backend backend_traffic in service and discovery status JSON
• refactor route traffic rendering into a shared API helper used by both service and status routes
• document the weighted rollout visibility surface in the routing docs and example

Validation

• YOQ_SKIP_SLOW_TESTS=1 zig build test
• Result: 1760 passed; 26 skipped; 0 failed