chore: replace @shopify/react-native-performance with RN's built-in startup timing

[janicduplessis]

What changed (plus any additional context for devs)

Drops @shopify/react-native-performance (unmaintained since Aug 2022, has an open Kotlin override-variance compile bug on RN 0.81+ that we're patching around) and replaces it with React Native's built-in performance.rnStartupTiming API. The performance.report and Performance Wallet Initialize Time Amplitude events are preserved.

performance.rnStartupTiming.startTime is the same shape of signal we get today — a process-start timestamp captured natively before the JS bundle loads — but exposed via RN's own StartupLogger singleton instead of a third-party module + custom patch. RN itself doesn't fire the marker by default — the host app has to fire APP_STARTUP_START from native code, which is exactly what the Shopify lib's ReactNativePerformance.onAppStarted() was doing.

Native wiring

• Android — MainApplication.kt: ReactMarker.logMarker(ReactMarkerConstants.APP_STARTUP_START) as the first line of onCreate. Pre-bridge calls are buffered in sNativeReactMarkerQueue and replayed once JNI loads (ReactMarker.java).
• iOS — AppDelegate.swift: RCTPerformanceLogger().markStart(for: .appStartup) from application(_:didFinishLaunchingWithOptions:). markStart fires ReactMarker::logMarkerDone(APP_STARTUP_START) which writes to a static C++ singleton, so the throwaway logger doesn't need to be retained. The bridging header gains <React/RCTPLTag.h> and <React/RCTPerformanceLogger.h> so Swift can see the API.

JS

• src/performance/start-time/index.ts reads performance.rnStartupTiming.startTime, falling back to performance.now() when absent (e.g. in jest, where the TurboModule isn't wired).
• src/performance/tracking/index.ts switches Date.now() → performance.now() everywhere reports anchor time. With the Shopify lib we were mixing wall-clock (Date.now() for segment math) and mach time (nativeStartupTimestamp from the lib's iOS captures) — that's a latent bug the new wiring fixes incidentally, since rnStartupTiming.startTime and performance.now() are both steady_clock on both platforms.
• src/App.tsx drops <PerformanceProfiler> and the onReportPrepared callback.
• WelcomeScreen and WalletScreen drop their <PerformanceMeasureView> wraps.

Where the "TTI" signal comes from now

The Shopify lib's <PerformanceProfiler> had a render-stability heuristic to detect "the screen has settled" and fire its onReportPrepared callback — that's where the tti segment of performance.report was logged. The replacement signal is splash-hide: useHideSplashScreen.ts already gates a once-only block on first hide; that's where we now log the tti segment, finish initialScreenInteractiveRender, and emit appStartup.

This is slightly earlier than the prior heuristic on WalletScreen (where <PerformanceMeasureView interactive={!isLoadingUserAssets}> waited for assets to load before reporting interactive). For an existing wallet, expect reported TTI to drop by the asset-load delta — closer to "first paint" than "fully populated screen". This was a deliberate trade-off: simpler signal, no third-party heuristic, easy to reason about. If we want the asset-load gating back later, that's one extra useEffect in the screen.

Build

• @shopify/react-native-performance removed from package.json + yarn.lock.
• patches/@shopify+react-native-performance+4.1.2.patch deleted.
• ios/Podfile.lock regenerated. Drops ReactNativePerformance. Side-effect: picks up an RNFlashList 1.8.2 → 1.8.3 and react-native-udp checksum drift that were already inconsistent between develop's package.json and Podfile.lock — pod install resolved the mismatch on the way through.

Screen recordings / screenshots

N/A — no visual changes.

What to test

• Cold start with no wallet → Welcome screen renders, animation plays, performance.report arrives in Amplitude.
• Cold start with a wallet → Wallet screen renders, asset list scrolls, performance.report arrives in Amplitude. (Expect TTI value to be slightly lower than baseline — see "Where the TTI signal comes from now" above.)
• Onboarding: Welcome → create wallet → Wallet screen, no native crashes.
• Splash hides on first WalletScreen / WelcomeScreen mount; poolboy easter-egg sound still plays once.
• WalletConnect: deeplink connect + sign roundtrip.
• Wallet import from seed phrase end-to-end.
• Android builds cleanly (yarn android) — ReactMarker.logMarker(APP_STARTUP_START) is the load-bearing change here.
• iOS: bundle exec pod install succeeds (drops react-native-performance), yarn ios succeeds. Confirm the bridging-header import resolves (<React/RCTPLTag.h>, <React/RCTPerformanceLogger.h>).
• In Amplitude: confirm performance.report events still arrive, tti segment is non-zero and roughly tracks baseline (allowing for the <PerformanceMeasureView> semantic change above).