ocs_doc_api: Phase 3 attributes + nested block traversal

- SetAttribute op + GetAttributes/GetBlockEntities queries (append-only).
- backend set_attribute (add/update Insert attribute), attributes (tag,value pairs),
  block_entities (read-only traversal via BlockRecord.entity_handles -> id+kind+bounds).
- facade Entity::set_attribute/attributes, Document::block_entities.
- spec Insert methods; api_reference + binding_schema regenerated; mocks + baseline.
- Query enum: drop Copy (GetBlockEntities carries String).
- Test phase3_attributes_and_block_traversal -> 608 host lib + 3 consistency green.

crates/ocs_doc_api/src/ops.rs

@@ -169,6 +169,7 @@ impl crate::gen::Operation {
             SetTextContent { .. } => "SetTextContent",
             CreateHatch(_) => "CreateHatch",
             CreateDimensionLinear(_) => "CreateDimensionLinear",
+            SetAttribute { .. } => "SetAttribute",
         }
     }
 }