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/gen/api_reference.md

@@ -145,6 +145,11 @@ Block reference. Place an existing BlockRecord by name; transform moves its inse
 
 - **create_insert**(`block_name: &str, insert_point: [f64; 3], scale: f64, rotation: f64`) -> `Entity`
 
+### Methods
+
+- **attributes**(``) -> `Vec<(String, String)>` — query `GetAttributes`
+- **set_attribute**(`tag: &str, value: &str`) -> `()` — op `SetAttribute`
+
 ## `Entity` (acadrust `Viewport`, collection `entities`)
 
 Paper-space viewport: a width×height window at center looking at view_target with view_height zoom. bounds = center ± w/2,h/2; transform moves center.