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/examples/common/mod.rs

@@ -99,6 +99,12 @@ impl DocApiBackend for MockBackend {
         self.curves.insert(id, spec.clone());
         Ok(id)
     }
+    fn add_insert(&mut self, spec: &ocs_doc_api::ops::InsertSpec) -> ApiResult<ObjectId> {
+        if spec.block_name.is_empty() {
+            return Err(ApiError::validation("CreateInsert", "empty block name"));
+        }
+        Ok(self.alloc("Insert"))
+    }
     fn add_vertex(&mut self, id: ObjectId, _at: usize, _point: [f64; 3]) -> ApiResult<()> {
         if self.entity_exists(id) {
             Ok(())