Move the 503-line host-independent entity-conversion module (acadrust EntityType
<-> DTO conversions, transform_entity_geometry, entity_to_profile_curves,
bulge_arc_segment, entity_bounds, entity_kind_name, points_aabb) from
src/app/doc_api_convert.rs into crates/ocs_doc_api/src/convert.rs (host feature).
The host file is now a thin re-export (pub use ocs_doc_api::convert::*) so all
existing crate::app::doc_api_convert::* paths keep working unchanged. Zero
coupling risk (the module only touches acadrust + crate DTOs).
623 host lib + 8 crate + 3 consistency green.