Async picker and completion protocol

The interactive picker and catalog completion engine use independently owned, versioned request/response envelopes. Both are protocol version 1 and have a frozen_major reader policy: only version 1 is readable. There is no pre-envelope wire format to migrate, so missing, expired, and future versions fail closed with a validation diagnostic. A future compatible shape requires a new major protocol version and an explicit migration range.

Every request has a strictly increasing request_id, a positive bounded deadline, and a bounded result limit. Picker requests additionally bound the query, item count, total payload, each JSON value, and JSON nesting. Completion requests bound the input, cursor boundary, result count, and deadline.

Workers compare each in-flight request against the newest ID and accept a versioned cancellation envelope. Results are published and consumed only when their request ID is still current; a delayed result can therefore never replace the display for newer input. Cancellation and deadline expiry have explicit typed response outcomes rather than partial selection or completion data.

The exact descriptors and hashes are recorded in protocol-freeze-v1.json under the picker and completion contracts.