Add Startup Trace Instrumentation

[maugauwi-hash]

Area: Performance
Priority: Low
Estimated Time: 1.5 hours

Description:
There's no apparent structured startup-performance instrumentation (Android Macrobenchmark /
androidx.tracing, iOS os_signpost) to give #316's cold-start work an objective before/after
measurement, or to catch a future regression before it ships.

Tasks:

• Add Macrobenchmark-based startup tracing to the Android CI pipeline
• Add os_signpost-based startup tracing for iOS, viewable via Instruments
• Add a CI check that flags a startup-time regression beyond a defined threshold

[glendell03]

I have no information about which repository or codebase this issue belongs to, so I can't verify current implementation state. I'll write the comment based on the issue text alone.

Since no source-reading findings were provided, I'll draft a general but specific comment referencing the issue's own details.

I'd like to take this on. To scope it: I'd add a Macrobenchmark module driving StartupTimingMetric against the Android app's launch activity, wired into CI as a dedicated job; on iOS, wrap app-launch and first-frame in os_signpost intervals so they're visible in Instruments' Points of Interest track. For the regression gate, my inclination is a fixed threshold (e.g. flag if median cold-start exceeds baseline by >10%) rather than a statistical/percentile check, since it's simpler to reason about in CI logs — happy to align on the exact threshold and baseline-storage approach before implementing, since that affects both pipelines. Would tie the "before" measurement to #316's changes if still relevant. Please assign me.