Complete Viewport: layout-tab canvas oracle, unique ids, derived scale

The shared driver now activates the layout that owns a viewport and requires
a drawn frame. The host assigns a unique id of at least 2 and derives
custom_scale as height / view_height on every scripted create or edit, as
MVIEW does, so custom_scale is read-only. A control showed the default id 0
is not invisible, so the id matters for uniqueness.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

plugins/opencad-python/entity_manifest.json

@@ -752,6 +752,10 @@
         },
         "height": {
           "required": true
+        },
+        "custom_scale": {
+          "read_only": true,
+          "_comment": "Derived by the host as height / view_height, like MVIEW."
         }
       }
     },