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/spec/entities.toml

@@ -252,6 +252,21 @@ name = "create_insert"
 args = [{ name = "block_name", ty = "&str" }, { name = "insert_point", ty = "[f64; 3]" }, { name = "scale", ty = "f64" }, { name = "rotation", ty = "f64" }]
 op = "CreateInsert"
 
+# ============================================================================
+# PAPER-SPACE FAMILY (phase 4) — viewports
+# ============================================================================
+[[family]]
+name = "Viewport"
+acadrust_variant = "Viewport"
+handle = "Entity"
+collection = "entities"
+description = "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."
+
+[[family.constructor]]
+name = "create_viewport"
+args = [{ name = "center", ty = "[f64; 3]" }, { name = "width", ty = "f64" }, { name = "height", ty = "f64" }, { name = "view_target", ty = "[f64; 3]" }, { name = "view_height", ty = "f64" }]
+op = "CreateViewport"
+
 # ============================================================================
 # Cross-cutting ops (generic handles + bulk) — documented in build.rs
 # ============================================================================