feat(browser): Emit web vitals as streamed spans when span streaming is enabled

Add non-standalone web vital spans that flow through the v2 span streaming
pipeline (afterSpanEnd -> captureSpan -> SpanBuffer). Each web vital gets
`browser.web_vital.<metric>.value` attributes and span events for measurement
extraction. Spans have meaningful durations showing time from navigation start
to the web vital event (except CLS which is a score, not a duration).

New tracking functions: trackLcpAsSpan, trackClsAsSpan, trackInpAsSpan,
trackTtfbAsSpan, trackFcpAsSpan, trackFpAsSpan — wired up in
browserTracingIntegration.setup() when hasSpanStreamingEnabled(client).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: logaretm

packages/browser-utils/src/index.ts

@@ -4,6 +4,7 @@ export {
   addTtfbInstrumentationHandler,
   addLcpInstrumentationHandler,
   addInpInstrumentationHandler,
+  addFcpInstrumentationHandler,
 } from './metrics/instrument';
 
 export {
@@ -20,6 +21,15 @@ export { startTrackingElementTiming } from './metrics/elementTiming';
 
 export { extractNetworkProtocol } from './metrics/utils';
 
+export {
+  trackClsAsSpan,
+  trackFcpAsSpan,
+  trackFpAsSpan,
+  trackInpAsSpan,
+  trackLcpAsSpan,
+  trackTtfbAsSpan,
+} from './metrics/webVitalSpans';
+
 export { addClickKeypressInstrumentationHandler } from './instrument/dom';
 
 export { addHistoryInstrumentationHandler } from './instrument/history';