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/executor.rs

@@ -176,6 +176,13 @@ pub fn apply_op<B: DocApiBackend>(b: &mut B, op: Operation) -> ApiResult<Receipt
             b.finalize_op();
             OpOutcome::Deleted(ids.clone())
         }
+        Operation::CreateInsert(spec) => {
+            // Validate the block name exists before committing (no rollback needed).
+            b.push_undo(name);
+            let id = b.add_insert(spec)?;
+            b.finalize_op();
+            OpOutcome::NewId(id)
+        }
     };
     Ok(Receipt {
         outcome: Some(outcome),