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

crates/ocs_doc_api/src/gen/binding_schema.json

@@ -194,6 +194,13 @@
       "handle": "Entity",
       "methods": [],
       "name": "Insert"
+    },
+    {
+      "acadrust_variant": "Viewport",
+      "collection": "entities",
+      "handle": "Entity",
+      "methods": [],
+      "name": "Viewport"
     }
   ],
   "layouts": {

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),
 }