Promote HTTP routing to first-class config

[kacy]

Summary

• promote [service..http_proxy] to a documented first-class service config
• add configurable HTTP routing listener flags to yoq serve and yoq init-server
• persist a service-level HTTP proxy target port and use it for proxy upstream selection and VIP steering
• add an HTTP routing example and guide updates

Validation

• ran zig fmt on the touched Zig files
• ran env YOQ_SKIP_SLOW_TESTS=1 ZIG_GLOBAL_CACHE_DIR=.zig-global-cache ZIG_LOCAL_CACHE_DIR=.zig-local-cache zig build test
• one completed suite run reached 1679 passed; 10 skipped; 7 failed, all in HTTP routing and status assertions touched by this work
• follow-up test fixes were applied for those assertion failures, but the final rerun did not return a stable completion status through the shell session