feat(annotative): synthesize per-object contexts + editable UI

Make objects genuinely annotative — carrying real per-scale
representations that interoperate with other CAD apps — rather than just
a native flag. `create_annotation_context` synthesizes the extension-dict
chain (AcDbContextDataManager -> ACDB_ANNOTATIONSCALES -> per-scale leaf)
from the entity's placement; the editable Annotative toggle now covers
Text and block references as well as MText/MLeader; and a new
"Annotation Object Scale" dialog (OBJECTSCALE) adds/removes an object's
per-scale memberships. Requires the acadrust ObjectContextData encoder
(Cargo.lock bump).

Also fixes a render bug where objects carrying an *empty*
ACDB_ANNOTATIONSCALES (a single-representation marker with no per-scale
reps) were treated as annotative and (mis)scaled by the annotation factor
in scaled paper viewports, ballooning the text: an object is now
annotative-by-context only when its scale collection is non-empty, so
such objects render at their base geometry.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/view/mod.rs

@@ -1574,6 +1574,7 @@ impl OpenCADStudio {
             LayerDeleteWarning => (440, 200),
             Aliases => (480, 520),
             ScaleManager => (520, 360),
+            AnnoObjectScale => (360, 420),
         };
         Some((w as f32 + EXTRA_W, h as f32 + EXTRA_H))
     }