Surface bundle load time as a standalone transaction measurement

[antonis]

The "JS Bundle Execution Before React Root" child span on app start already captures JS bytecode load + evaluation duration (from getBundleStartTimestampMs() to rootComponentCreationTimestampMs). However, this duration is only available as a child span — it's not emitted as a top-level transaction measurement, unlike app_start_cold/app_start_warm and time_to_initial_display/time_to_full_display which get both spans and measurements.

Surfacing it as a measurement (e.g. js_bundle_execution) would make it independently queryable and dashboardable.

Implementation

In appStart.ts attachAppStartToTransactionEvent(), after the existing app start measurement is set (~line 646), add:

if (jsExecutionSpanJSON) {
  setSpanDurationAsMeasurementOnTransactionEvent(event, 'js_bundle_execution', jsExecutionSpanJSON);
}

This works for both standalone and non-standalone app start modes since both go through attachAppStartToTransactionEvent.

Caveats

• The measurement depends on rootComponentCreationTimestampMs being set (via Sentry.wrap()). If not set, the span is a zero-duration marker — skip the measurement in that case.
• The measurement key (js_bundle_execution) needs alignment with the product/backend team so it's recognized, indexed, and displayed in the UI.
• Coordinates with the Standalone App Start [React Native] project (feat(tracing): Decouple app start data from navigation transaction lifecycle #5839, Add API to extend app start (React Native) #6303, Make Standalone App Start default #6304).

Scope

JS only. ~5 lines of code + tests.

[linear-code]

RN-681