Measure a slow UI interaction

usePerf records spans — one completed operation with a duration, a name, and free-form attributes — to a JSONL file the Go side owns (ADR ui-performance-spans-are-recorded-to-jsonl). Every sample is kept; nothing is aggregated at write time, so the outlier that is usually the actual bug survives to the analysis step.

Pick the right tool first:

• A slow interaction — a click that takes a beat, a view that stutters on open, a fetch whose cost is unclear — is this skill. It measures what the user actually waits for, RPC and render included.
• A hot Go path — CPU in the producer, the flow engine, a store query — is pprof (ADR pprof-debug-endpoint), enabled with development.pprof.enabled and mounted at /debug/pprof/ on the loopback server. These numbers are not comparable; a span includes time pprof never sees.

1. Confirm recording is on