iOS ETTrace performance

Tool: https://github.com/EmergeTools/ETTrace (brew install emergetools/homebrew-tap/ettrace, then ettrace --help). Pair with ios-debugger-agent for build, launch, and UI driving.

• One trace per user-visible flow. Link ETTrace.xcframework into the exact app target temporarily and remove it afterward.
• Do not draw conclusions from an unsymbolicated trace. Provide UUID-matched dSYMs for the app and embedded first-party frameworks via --dsyms.
• Run ettrace in a TTY. Only one instrumented simulator app at a time (fixed localhost port).
• Analyze only the processed output_<thread>.json files, and copy them right away because the next run overwrites them. Ignore output.json.
• Report flow, build, simulator runtime, top stacks with weights, symbolication completeness, and caveats. Compare before and after only for the same flow and setup.