feat(text): MTEXT background fill / mask + text frame, editable in Properties

Render the MTEXT background (never drawn before — the flags were only shown
read-only in the panel) and make it editable.

- Text arm emits an opaque fill_tris rect behind the glyphs: flag 0x01 uses
  the entity background colour, 0x02 uses the canvas colour (a wipeout-style
  mask). Box = glyph bounds padded by (background_scale - 1) × text height.
- Text frame (flag 0x10) draws a closed rectangle in the text colour around
  the same box.
- Threaded `bg_color` into `tessellate()` (+ all callers and the
  MultiLeaderTess trait) so the mask can use the drawing-window colour.
- Properties: "Background mask" is a Off / Fill / Mask dropdown, "Background
  color" is its own colour picker (independent of the entity colour — new
  PropBgColor* messages; picking a colour switches to Fill mode), and "Text
  frame" is a checkbox toggle. Edits re-tessellate so the change shows live.

Renders via the face3d fill path, so the fill shows with FILLMODE on (same as
MLEADER background fill); the frame is a stroke and always shows.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/update/mod.rs

@@ -2214,6 +2214,36 @@ impl OpenCADStudio {
                 Task::none()
             }
 
+            Message::PropBgColorPickerToggle => {
+                let i = self.active_tab;
+                self.tabs[i].properties.bg_color_picker_open =
+                    !self.tabs[i].properties.bg_color_picker_open;
+                Task::none()
+            }
+
+            Message::PropBgColorChanged(color) => {
+                let i = self.active_tab;
+                let handles = self.property_target_handles(i);
+                if !handles.is_empty() {
+                    self.push_undo_snapshot(i, "CHPROP");
+                    for &handle in &handles {
+                        if let Some(acadrust::EntityType::MText(m)) =
+                            self.tabs[i].scene.document.get_entity_mut(handle)
+                        {
+                            m.background_color = color.clone();
+                            // Picking a colour turns the background on in Fill
+                            // mode (specific colour), preserving the frame bit.
+                            m.background_fill_flags = (m.background_fill_flags & !0x02) | 0x01;
+                        }
+                    }
+                    self.invalidate_property_targets(i, &handles);
+                    self.tabs[i].properties.bg_color_picker_open = false;
+                    self.tabs[i].dirty = true;
+                    self.refresh_properties();
+                }
+                Task::none()
+            }
+
             Message::PropColorPickerClose => {
                 let i = self.active_tab;
                 self.tabs[i].properties.color_picker_open = false;