ocs_doc_api: Phase 3 (block references)

- Operation::CreateInsert(InsertSpec) appended to the wire enum (append-only).
- backend add_insert validates the BlockRecord exists (unknown block -> Validation,
  no mutation); host inserts acadrust::entities::Insert with scale+rotation.
- Insert transform moves insert_point (transform_entity_geometry) +
  ensure_transformable includes Insert.
- facade EntityCollection::create_insert; spec Insert family; api_reference +
  binding_schema regenerated; mocks updated.
- Test phase3_create_insert_and_transform -> 599 host lib green.
- IMPLEMENTATION.md: Phase 3 done (attributes/traversal remaining).

crates/ocs_doc_api/src/gen/api_reference.md

@@ -87,6 +87,14 @@ B-rep solid. Host resolves bodies via the solid_models cache (lift-on-miss from
 
 - **create_xline**(`origin: [f64; 3], direction: [f64; 3]`) -> `XLine`
 
+## `Entity` (acadrust `Insert`, collection `entities`)
+
+Block reference. Place an existing BlockRecord by name; transform moves its insert point.
+
+### Constructors (`doc.entities()`)
+
+- **create_insert**(`block_name: &str, insert_point: [f64; 3], scale: f64, rotation: f64`) -> `Entity`
+
 ## Generic methods (every handle)
 
 Every typed handle (`Solid`, `Line`, `Circle`, `Polyline`, `Point`, `Entity`) is a