Surface bundle load time as a standalone transaction measurement #6335

The original goal was to surface JS bundle load time more prominently, since it was buried as a child span two levels deep under a navigation transaction.

The Standalone App Start work has addressed this:

• feat(tracing): Add standalone app start transaction #6359 — Standalone app.start transaction: "JS Bundle Execution Before React Root" is now a direct child span of a dedicated root transaction, not buried under a navigation transaction
• feat(tracing): Emit app.vitals.start.screen on standalone app start #6369 — Added app.vitals.start.screen attribute for screen context
• feat(tracing): Add extend app start API for standalone app start #6392 — Added extendAppStart()/finishExtendedAppStart() API for user-instrumented work

The bundle load time data is available as a span in the trace waterfall and can be queried via span-based queries in Explore.