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/backend.rs

@@ -35,6 +35,10 @@ pub trait DocApiBackend {
     /// Add a 2D curve entity; returns the fresh `ObjectId`.
     fn add_curve(&mut self, spec: &Curve2Spec) -> ApiResult<ObjectId>;
 
+    /// Add a block reference (`INSERT`). Validates `block_name` exists; returns
+    /// the fresh `ObjectId`. `Validation` if the block is unknown.
+    fn add_insert(&mut self, spec: &crate::ops::InsertSpec) -> ApiResult<ObjectId>;
+
     /// Remove any first-level entity by id. Returns `false` if absent; returns
     /// `Err` if the entity exists but cannot be removed (e.g. locked layer).
     fn remove_entity(&mut self, id: ObjectId) -> ApiResult<bool>;