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/pipeline/circle_gpu.rs

@@ -588,6 +588,7 @@ mod tests {
         let h = doc.add_entity(donut).unwrap();
         let entity = doc.get_entity(h).unwrap();
         let selected = rustc_hash::FxHashSet::default();
+        let reference_only = rustc_hash::FxHashSet::default();
         let wires = crate::scene::convert::tess::tessellate_entity(
             &doc,
             &selected,
@@ -600,6 +601,7 @@ mod tests {
             None,
             None,
             false,
+            &reference_only,
         );
         assert_eq!(
             wires.len(),