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 <noreply@anthropic.com>

src/entities/insert.rs

@@ -239,6 +239,7 @@ pub(crate) fn append_insert_attribute_wires(
             sub_lw_px,
             anno_scale,
             None,
+            bg_color,
         );
         // Use the INSERT's handle so selection / picking groups attribute
         // text with the parent insert instead of treating it as a stray text.

src/entities/mtext.rs

@@ -204,7 +204,6 @@ fn properties(t: &MText, text_style_names: &[String]) -> Vec<PropSection> {
     // Absolute line-space distance for single spacing (~1.66 * height) scaled
     // by the line-spacing factor.
     let line_space_distance = t.height * 1.666_666_666_666_667 * t.line_spacing_factor;
-    let bg_mask_on = (t.background_fill_flags & 0x03) != 0;
     let text_frame_on = (t.background_fill_flags & 0x10) != 0;
     vec![
         PropSection {
@@ -280,18 +279,38 @@ fn properties(t: &MText, text_style_names: &[String]) -> Vec<PropSection> {
                         _ => "At least",
                     },
                 ),
-                ro(
-                    "Background mask",
-                    "background_mask",
-                    if bg_mask_on { "On" } else { "Off" },
-                ),
+                Property {
+                    label: "Background mask".into(),
+                    field: "background_mask",
+                    value: PropValue::Choice {
+                        selected: if t.background_fill_flags & 0x01 != 0 {
+                            "Fill".into()
+                        } else if t.background_fill_flags & 0x02 != 0 {
+                            "Mask".into()
+                        } else {
+                            "Off".into()
+                        },
+                        options: ["Off", "Fill", "Mask"]
+                            .into_iter()
+                            .map(str::to_string)
+                            .collect(),
+                    },
+                },
+                Property {
+                    label: "Background color".into(),
+                    field: "background_color",
+                    value: PropValue::ColorChoice(t.background_color.clone()),
+                },
                 edit("Defined width", "rect_w", t.rectangle_width),
                 edit("Defined height", "rect_h", t.rectangle_height.unwrap_or(0.0)),
-                ro(
-                    "Text frame",
-                    "text_frame",
-                    if text_frame_on { "On" } else { "Off" },
-                ),
+                Property {
+                    label: "Text frame".into(),
+                    field: "text_frame",
+                    value: PropValue::BoolToggle {
+                        field: "text_frame",
+                        value: text_frame_on,
+                    },
+                },
                 ro(
                     "Columns",
                     "columns",
@@ -336,6 +355,27 @@ fn apply_geom_prop(t: &mut MText, field: &str, value: &str) {
             }
             return;
         }
+        "background_mask" => {
+            // Clear both fill bits, then set the chosen one. 0x01 = use the
+            // background-fill colour, 0x02 = use the drawing-window colour (mask).
+            t.background_fill_flags &= !0x03;
+            match value {
+                "Fill" => t.background_fill_flags |= 0x01,
+                "Mask" => t.background_fill_flags |= 0x02,
+                _ => {}
+            }
+            return;
+        }
+        "text_frame" => {
+            // Rendered as a checkbox (BoolToggle) → the toggle sends "toggle";
+            // flip the frame bit. Accept explicit "On"/"Off" too for safety.
+            match value {
+                "toggle" => t.background_fill_flags ^= 0x10,
+                "On" => t.background_fill_flags |= 0x10,
+                _ => t.background_fill_flags &= !0x10,
+            }
+            return;
+        }
         _ => {}
     }
     let Some(v) = crate::entities::common::parse_f64(value) else {

src/entities/multileader.rs

@@ -1071,6 +1071,7 @@ pub trait MultiLeaderTess {
         line_weight_px: f32,
         anno_scale: f32,
         world_per_pixel: Option<f32>,
+        bg_color: [f32; 4],
     ) -> Vec<crate::scene::model::wire_model::WireModel>;
 }
 
@@ -1084,6 +1085,7 @@ impl MultiLeaderTess for MultiLeader {
         line_weight_px: f32,
         anno_scale: f32,
         world_per_pixel: Option<f32>,
+        bg_color: [f32; 4],
     ) -> Vec<crate::scene::model::wire_model::WireModel> {
         use crate::scene::convert::tessellate::{
             append_arrow, arrow_from_block, color_or_inherit, tessellate, ArrowKind, DimGeom,
@@ -1354,6 +1356,7 @@ impl MultiLeaderTess for MultiLeader {
                         leader_lw_px,
                         1.0,
                         None,
+                        bg_color,
                     );
                     for w in &mut sub_wires {
                         w.name = name.clone();