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/facade.rs

@@ -128,6 +128,14 @@ impl Document {
         }
     }
 
+    /// Read-only traversal of a block definition's entities (ids + kinds + bounds).
+    pub fn block_entities(&self, block_name: &str) -> ApiResult<Vec<EntityView>> {
+        match self.session.one_query(Query::GetBlockEntities { block_name: block_name.to_string() })? {
+            QueryResult::BlockEntities(v) => Ok(v),
+            _ => Err(ApiError::Transport("unexpected block-entities result".into())),
+        }
+    }
+
     /// Batch of read-only queries in ONE round-trip (safe: no mutation/undo).
     /// The closure records queries on a [`QueryBatch`]; the results are returned
     /// in the same order as a [`QueryResults`] view the caller destructures.
@@ -289,6 +297,18 @@ impl Entity {
             _ => Err(ApiError::Transport("unexpected entity result".into())),
         }
     }
+    /// This insert's attributes as (tag, value) pairs. Insert-only.
+    pub fn attributes(&self) -> ApiResult<Vec<(String, String)>> {
+        match self.session.one_query(Query::GetAttributes { id: self.id })? {
+            QueryResult::Attributes(v) => Ok(v),
+            _ => Err(ApiError::Transport("unexpected attributes result".into())),
+        }
+    }
+    /// Set an attribute `value` for `tag` on this insert (adds if absent). One undo step.
+    pub fn set_attribute(&self, tag: &str, value: &str) -> ApiResult<()> {
+        self.session.apply_op(Operation::SetAttribute { id: self.id, tag: tag.to_string(), value: value.to_string() })?;
+        Ok(())
+    }
     pub fn as_solid(&self) -> Option<Solid> {
         // Typed downcast is validated by the view's kind.
         matches!(self.view().ok()?.kind.as_str(), "Solid3D").then(|| Solid::new(self.session.clone(), self.id))