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/backend.rs

@@ -39,6 +39,9 @@ pub trait DocApiBackend {
     /// the fresh `ObjectId`. `Validation` if the block is unknown.
     fn add_insert(&mut self, spec: &crate::ops::InsertSpec) -> ApiResult<ObjectId>;
 
+    /// Add a paper-space `VIEWPORT`; returns the fresh `ObjectId`.
+    fn add_viewport(&mut self, spec: &crate::ops::ViewportSpec) -> ApiResult<ObjectId>;
+
     /// Remove any first-level entity by id. Returns `false` if absent; returns
     /// `Err` if the entity exists but cannot be removed (e.g. locked layer).
     fn remove_entity(&mut self, id: ObjectId) -> ApiResult<bool>;