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/entity_coverage_policy.json

@@ -448,7 +448,6 @@
       "back_clip_z",
       "view_height",
       "twist_angle",
-      "custom_scale",
       "frozen_layers",
       "render_mode",
       "circle_sides",
@@ -611,7 +610,8 @@
       "default_lighting_type",
       "brightness",
       "contrast",
-      "ambient_color"
+      "ambient_color",
+      "custom_scale"
     ],
     "ViewBorder": [
       "version"