v0.18.136 — RUM navigation-timing milestones

Performance

• RUM navigation-timing milestones (BORD-999). Web Vitals' TTFB only covers responseStart, which hid the window between the first byte and first paint. /inbox reported a ~64 ms TTFB against a ~2.4 s FCP, making a server-side document stall look like client boot cost. The reporter now emits nav_response_end, nav_dom_interactive, and nav_dom_content_loaded, so the pre-paint window is attributable per release. Instrumentation only — no rendering change.