Complete MLine host model coverage

Maps vertices, flags, justification, normal, scale and style with host
validation and MLineStyle binding. The host recomputes vertex direction,
miter and element parameters on scripted create/edit, so scripts only supply
positions. staged_python_mline_lifecycle_over_real_ipc passes the full gate
including DWG and DXF reopen.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/entities/mline.rs

@@ -418,6 +418,54 @@ fn restore_shifted_mline_segment(
     }
 }
 
+/// Recompute the derived MLine state after a scripted create or edit:
+/// vertex direction/miter, element segment layout and the per-element miter
+/// offsets. Dash/cut parameters survive an edit unless a vertex moved.
+pub(crate) fn normalize_scripted_mline(
+    old: Option<&MLine>,
+    new: &mut MLine,
+    document: &acadrust::CadDocument,
+) -> Result<(), String> {
+    let style = resolved_mline_style(new, document)
+        .ok_or_else(|| format!("MLine style {:?} does not exist", new.style_name))?;
+    let unchanged = old.is_some_and(|old| {
+        old.vertices == new.vertices
+            && old.normal == new.normal
+            && old.flags == new.flags
+            && old.justification == new.justification
+            && old.scale_factor.to_bits() == new.scale_factor.to_bits()
+            && old.style_name == new.style_name
+            && old.style_handle == new.style_handle
+    });
+    if unchanged {
+        return Ok(());
+    }
+    let moved = old.is_none_or(|old| {
+        old.vertices.len() != new.vertices.len()
+            || old
+                .vertices
+                .iter()
+                .zip(&new.vertices)
+                .any(|(a, b)| a.position != b.position)
+    });
+    let elements = style.elements.len().max(1);
+    for vertex in &mut new.vertices {
+        if vertex.segments.is_empty() {
+            vertex.init_segments(elements);
+        } else if moved {
+            for segment in &mut vertex.segments {
+                segment.parameters.truncate(1);
+                segment.area_fill_parameters.clear();
+            }
+        }
+    }
+    if !rebuild_mline_geometry(new) {
+        return Err("MLine vertices are degenerate".into());
+    }
+    crate::modules::draw::draw::mline::sync_mline_element_parameters(new, style);
+    Ok(())
+}
+
 /// Resolve a multiline into styled parallel lines in WCS.
 pub fn resolved_mline_style<'a>(
     m: &MLine,