Map ViewBorder in the Python host model; record DXF kind blocker

Bounds, scale and rotation are writable with viewport and scale reference
rules. staged_python_view_border_lifecycle_over_real_ipc passes every gate
in DWG; a DXF DRAWINGVIEW reopens as an opaque entity, pinned by a canary.

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

crates/ocs_plugin_api/build.rs

@@ -795,6 +795,7 @@ fn generate_entity_coverage(out_dir: &Path, registry: &TypeRegistry) {
                     | ("Wipeout", _)
                     | ("Underlay", _)
                     | ("Viewport", _)
+                    | ("ViewBorder", _)
                     | (
                         "MLine",
                         "flags"