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/ops_gen.rs

@@ -6,7 +6,7 @@
 use serde::{Deserialize, Serialize};
 
 use crate::id::ObjectId;
-use crate::ops::{BoolOp, Curve2Spec, EntitySpec, InsertSpec, PlacementSpec, SolidPrimitive};
+use crate::ops::{BoolOp, Curve2Spec, EntitySpec, InsertSpec, PlacementSpec, SolidPrimitive, ViewportSpec};
 
 /// A typed write operation (plan §5). Each variant is ONE atomic host call = one
 /// undo step. Append new variants at the END only.
@@ -29,4 +29,6 @@ pub enum Operation {
     DeleteMany(Vec<ObjectId>),
     // containers
     CreateInsert(InsertSpec),
+    // paper-space
+    CreateViewport(ViewportSpec),
 }