Expose L7 steering readiness reasons

[kacy]

Summary

• expose per-route steering readiness and blocked reasons in service and rollout status
• add a steering blocked-reason Prometheus family for operator visibility
• compute per-service steering blocked reasons from actual listener, mapper, and bridge prerequisites

Validation

• zig fmt on touched files
• timeout 45 env YOQ_SKIP_SLOW_TESTS=1 ZIG_GLOBAL_CACHE_DIR=.zig-global-cache ZIG_LOCAL_CACHE_DIR=.zig-local-cache zig build test -Doptimize=ReleaseSafe -- --test-filter "route handles GET /v1/services/{name}/proxy-routes"
• timeout 45 env YOQ_SKIP_SLOW_TESTS=1 ZIG_GLOBAL_CACHE_DIR=.zig-global-cache ZIG_LOCAL_CACHE_DIR=.zig-local-cache zig build test -Doptimize=ReleaseSafe -- --test-filter "route handles /v1/status?mode=service_rollout GET"
• timeout 45 env YOQ_SKIP_SLOW_TESTS=1 ZIG_GLOBAL_CACHE_DIR=.zig-global-cache ZIG_LOCAL_CACHE_DIR=.zig-local-cache zig build test -Doptimize=ReleaseSafe -- --test-filter "handleMetricsPrometheus exposes service rollout metrics"