Count what extension exchanges cost in the bench output

NAPDOCK_BENCH lines gain an ext object with requests sent, request and frame sizes, and time spent building the snapshot, encoding, waiting for the worker, decoding and applying frames. Process::measured() in extension_host reports each exchange’s frame size, wait and decode time, and apps/native/examples/exchange_cost.rs times the app’s check and encode of saved requests.

The measurements showed that validating once, keyed patches, worker push and app-side list scrolling would save nothing measurable. The only cost that grows is the session snapshot: about 0.8 MB at the 2,000-tool cap. Session deltas were left until session_ms can be measured on a large live session. No pipeline behavior changes.