057f3b9: Emit FCP and TTFB Core Web Vitals from the browser RUM SDK. The bundled
web-vitals library already exports onFCP and onTTFB; initWebVitals
now registers callbacks for them alongside the existing LCP / INP / CLS / FID
ones. Each new metric lands as a webvitals span with the value on a single
attribute (fcp or ttfb), matching the existing pattern.
