Add a query op to the headless server: list entities with handle, type,
layer and basic geometry (Line start/end, Circle/Arc centre+radius, Point
location), filtered by type and/or layer and capped by limit. Closes the
observe half of an agent's act/observe loop. ocs.py gains ocs.query().
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>