Decide Insert and Polyline: Insert creatable by block name, Polyline update-only

Insert can be created by naming an existing ordinary block; the host refuses
unknown blocks, model/paper space and containment cycles, and keeps legacy
inserts of missing blocks editable. The legacy Polyline is update-only with a
hint toward Polyline2D and Polyline3D because both writers convert it on
save. The generator gains update_only_message.

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

crates/ocs_plugin_api/entity_coverage_policy.json

@@ -175,6 +175,7 @@
     ],
     "Insert": [
       "layer",
+      "block_name",
       "insert_point",
       "x_scale",
       "y_scale",
@@ -488,9 +489,6 @@
     ]
   },
   "readable": {
-    "Insert": [
-      "block_name"
-    ],
     "Tolerance": [
       "dimension_style_handle"
     ],