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/commands/display.rs

@@ -780,6 +780,12 @@ impl OpenCADStudio {
                 }
             }
 
+            // OBJECTSCALE — open the Annotation Object Scale dialog for the
+            // selected object (add / remove its per-object scale representations).
+            "OBJECTSCALE" => {
+                return Some(Task::done(Message::AnnoObjectScaleOpen));
+            }
+
             // DATALINK <path.csv> — import a CSV file into a table placed at the
             // origin (one-time import; a live re-reading link is future work).
             cmd if cmd == "DATALINK" || cmd.starts_with("DATALINK ") => {
@@ -873,7 +879,7 @@ impl OpenCADStudio {
                 }
             }
 
-            "POINTCLOUDATTACH" | "RECAP" | "SYNCPVIEWPORTS" | "UNDERLAYLAYERS" | "OBJECTSCALE"
+            "POINTCLOUDATTACH" | "RECAP" | "SYNCPVIEWPORTS" | "UNDERLAYLAYERS"
             | "UOSNAP" => {
                 self.command_line
                     .push_info(&format!("{cmd}: not yet implemented."));