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/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();