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>

crates/ocs_plugin_api/src/entity_coverage.rs

@@ -2942,7 +2942,6 @@ mod tests {
                 "Viewport.back_clip_z".to_owned(),
                 "Viewport.view_height".to_owned(),
                 "Viewport.twist_angle".to_owned(),
-                "Viewport.custom_scale".to_owned(),
                 "Viewport.frozen_layers".to_owned(),
                 "Viewport.render_mode".to_owned(),
                 "Viewport.circle_sides".to_owned(),