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/cache/block_cache.rs

@@ -772,6 +772,9 @@ fn tessellate_sub_local(
     // top-level entities, applied per-defn.
     let wires_out = if let EntityType::Dimension(dimension) = sub {
         use crate::entities::dimension::DimensionTess;
+        // Block-definition members never carry the reference-only
+        // designation (it targets top-level geometry — see the API v9
+        // known-limit note), so the block cache tessellates with an empty set.
         dimension.tessellate(
             doc,
             h,
@@ -784,6 +787,7 @@ fn tessellate_sub_local(
             bg_color,
             None,
             None,
+            &HashSet::default(),
         )
     } else {
         tessellate::tessellate(