fix(blocks): enforce constant attributes in ATTEDIT and restore ATTDEFs on explode

Enforce constant attribute flag in ATTEDIT dialog by presenting constant attributes as disabled and ignoring modifications. When exploding an insert, restore block-definition attribute definitions as transformed ATTDEF entities while dropping instance attribute values.

src/ui/window/attribute_editor.rs

@@ -59,6 +59,8 @@ pub struct AttrRow {
     pub color: AcadColor,
     pub linetype: String,
     pub line_weight: LineWeight,
+    /// Constant attributes cannot be edited.
+    pub is_constant: bool,
 }
 
 /// Combined justification options (horizontal × vertical), matching the
@@ -215,7 +217,24 @@ fn field_style(theme: &Theme, status: text_input::Status) -> text_input::Style {
     }
 }
 
+fn disabled_field_style(theme: &Theme, _status: text_input::Status) -> text_input::Style {
+    let palette = theme.palette();
+    text_input::Style {
+        background: Background::Color(palette.background.weak.color),
+        border: Border {
+            color: palette.background.neutral.color,
+            width: 1.0,
+            radius: 3.0.into(),
+        },
+        icon: palette.background.base.text.scale_alpha(0.38),
+        placeholder: palette.background.base.text.scale_alpha(0.38),
+        value: palette.background.base.text.scale_alpha(0.48),
+        selection: palette.primary.base.color.scale_alpha(0.2),
+    }
+}
+
 /// One `label : widget` row with a fixed-width label column.
+
 fn field_row<'a>(
     label: Cow<'static, str>,
     widget: Element<'a, Message>,
@@ -469,14 +488,21 @@ fn attribute_tab<'a>(
     }
 
     let sel = selected.min(rows.len() - 1);
-    let value_box = text_input("", rows[sel].value.as_str())
-        .on_input(move |v| Message::AttrEditorInput { idx: sel, value: v })
-        .on_submit(Message::AttrEditorApply)
-        .style(field_style)
+    let is_constant = rows[sel].is_constant;
+    let mut value_box = text_input("", rows[sel].value.as_str())
         .size(13)
         .padding([4, 6])
         .width(width);
 
+    if is_constant {
+        value_box = value_box.style(disabled_field_style);
+    } else {
+        value_box = value_box
+            .on_input(move |v| Message::AttrEditorInput { idx: sel, value: v })
+            .on_submit(Message::AttrEditorApply)
+            .style(field_style);
+    }
+
     column![
         head,
         scrollable(list).width(width).height(height),