ocs_doc_api: Phase 3-ii — typed AttributeDefinition create (Phase 3 complete)

- CreateAttributeDefinition(AttributeDefinitionSpec) op (append-only).
- backend add_attribute_definition (ATTDEF tag/prompt/default/insertion/height/rotation).
- facade create_attribute_definition; entity_kind_name AttributeDefinition/AttributeEntity.
- spec AttributeDefinition family; api_reference + binding_schema regenerated; mocks + baseline.
- Test phase3ii_create_attribute_definition -> 614 host lib + 3 consistency green.

crates/ocs_doc_api/src/backend.rs

@@ -72,6 +72,9 @@ pub trait DocApiBackend {
     /// Add a 3-point angular `DIMENSION`; returns the fresh `ObjectId`.
     fn add_dimension_angular(&mut self, spec: &crate::ops::DimensionAngularSpec) -> ApiResult<ObjectId>;
 
+    /// Add an `ATTDEF` (in-block attribute definition); returns the fresh `ObjectId`.
+    fn add_attribute_definition(&mut self, spec: &crate::ops::AttributeDefinitionSpec) -> ApiResult<ObjectId>;
+
     /// A dimension's measured value (distance for linear/radius, degrees for angular).
     fn dimension_measurement(&self, id: ObjectId) -> ApiResult<f64>;