refactor(scene): reference-only designation becomes a scene-scoped set (API v9)

Per the upstream review of the XDATA-based storage: the host no longer writes
its own state into the saved file. The marker record under the host-owned OCS
APPID, the on-first-use APPID registration and the verbatim-EED rewrite
handling are removed; the single source of truth is a scene-scoped in-memory
set of entity handles (Scene::reference_only) that dies with the drawing tab,
so a reopened file starts undesignated and the owning plugin re-marks on the
document-opened notification.

All behaviour sites are unchanged, driven by the set: the wire stamp moves
from an XDATA read to a set lookup threaded through the tessellation
pipelines (tessellate_entity and the dimension text path take the handle set
as a plain parameter), and the pick filters, selection retains and
update/erase refusals read Scene::is_reference_only.

Undo keeps the entity-write contract: set mutations are recorded as explicit
first-touch before/after values on each history delta (the parametric-
constraints channel pattern). A designation inside a plugin push_undo scope
joins it, a scope-less change is one step of its own, an idempotent re-mark
records nothing, erasing a designated entity drops the set entry with the
entity, and undo/redo restores entity and designation together.

The round-trip test is inverted: reference_only_designation_is_not_written_
to_dwg_or_dxf now asserts the marker appears in neither format's bytes, that
no OCS APPID or marker record survives a reload, and that the designation
stays live in memory.

src/scene/selection.rs

@@ -1143,6 +1143,12 @@ impl Scene {
                     let before = self.document.objects.get(&manager_handle).cloned();
                     self.record_undo_object_before(manager_handle, before);
                 }
+                // The designation set is scene state, not document state: a
+                // designated entity's set entry joins the same first-touch
+                // recording so undo restores entity and designation together.
+                if self.reference_only.is_marked(h) {
+                    self.record_undo_reference_designation_before(h);
+                }
             }
             self.delete_solid_history(h);
             if let Some(settings_handle) = settings_handle {
@@ -1161,6 +1167,9 @@ impl Scene {
             }
             self.remember_removed_cache_categories(h);
             self.document.remove_entity_arc(h);
+            // The set entry dies with its entity (an absent entity is never
+            // reference-only). Undo restores it through the recorded channel.
+            self.reference_only.remove(h);
             selection_changed |= self.selected.remove(&h);
             self.selected_order.retain(|selected| *selected != h);
             if self.hover_highlight == Some(h) {