feat(props): make dim-line colour editable for leaders and dimensions

The "Dim line color" row on Leader and Dimension entities was
read-only: an earlier attempt wired it to Leader.override_color, which
acadrust never serialises, so the pick was lost on save. Store it
instead as a standard ACAD_DSTYLE per-object dimension-style override
(DXF code 176, an ACI index) through the existing dim_override codec,
so it round-trips through both DWG and DXF like the other dim
overrides. RGB picks collapse to the nearest ACI, matching the rest of
the dim-colour stack (dimension styles are index-only through the file
layer). The renderer prefers the override over the style's DIMCLRD.

The write branch is guarded to leaders and dimensions so a mixed
selection cannot stamp the override onto other entity types.

Bumps acadrust to c9fe982, which carries parsed XDATA onto dimensions
on DXF read (it was dropping common.extended_data), so the dimension
override persists on DXF save as well as DWG.

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

src/entities/dimension.rs

@@ -1290,11 +1290,18 @@ fn tessellate_dimension_inner(
     );
 
     // Per-spec colours: DIMCLRD (dim/arrows), DIMCLRE (ext), DIMCLRT (text).
-    // 0=ByBlock and 256=ByLayer fall through to entity_color.
+    // 0=ByBlock and 256=ByLayer fall through to entity_color. DIMCLRD also
+    // honours a per-object ACAD_DSTYLE override (code 176) so an edited
+    // dim-line colour renders even without touching the style.
     let dim_color = if selected {
         WireModel::SELECTED
     } else {
-        resolve_dim_color(style.map(|s| s.dimclrd).unwrap_or(0), entity_color)
+        let dim_clr = crate::entities::dim_override::int(
+            &dim.base().common.extended_data,
+            crate::entities::dim_override::DIMCLRD,
+        )
+        .unwrap_or_else(|| style.map(|s| s.dimclrd).unwrap_or(0));
+        resolve_dim_color(dim_clr, entity_color)
     };
     let ext_color = if selected {
         WireModel::SELECTED