ocs_doc_api: Phase 4 (paper-space viewports)

- Operation::CreateViewport(ViewportSpec) appended to the wire enum (append-only).
- backend add_viewport (center/width/height/view_target/view_height).
- Viewport bounds (center +- w/2,h/2), transform (moves center), ensure_transformable.
- facade EntityCollection::create_viewport; spec Viewport family; api_reference +
  binding_schema regenerated; mocks updated.
- Test phase4_create_viewport_bounds_transform -> 600 host lib green.
- IMPLEMENTATION.md: Phase 4 done (set_view/view-query remaining).

crates/ocs_doc_api/src/gen/api_reference.md

@@ -95,6 +95,14 @@ 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`
 
+## `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.
+
+### Constructors (`doc.entities()`)
+
+- **create_viewport**(`center: [f64; 3], width: f64, height: f64, view_target: [f64; 3], view_height: f64`) -> `Entity`
+
 ## Generic methods (every handle)
 
 Every typed handle (`Solid`, `Line`, `Circle`, `Polyline`, `Point`, `Entity`) is a