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/scene/convert/tess.rs

@@ -196,6 +196,7 @@ pub(crate) fn tessellate_entity(
             1.5,
             1.0,
             world_per_pixel,
+            bg_color,
         );
         let ab = entity_aabb(e);
         for w in &mut wires {
@@ -343,6 +344,7 @@ pub(crate) fn tessellate_entity(
             line_weight_px,
             anno_scale,
             world_per_pixel,
+            bg_color,
         );
         for w in &mut wires {
             w.aci = aci;
@@ -579,6 +581,7 @@ pub(crate) fn tessellate_entity(
                     sub_line_weight_px,
                     anno_scale,
                     world_per_pixel,
+                    bg_color,
                 );
                 for w in &mut wires {
                     w.name = h.value().to_string();
@@ -753,6 +756,7 @@ pub(crate) fn tessellate_entity(
         line_weight_px,
         anno_scale,
         world_per_pixel,
+        bg_color,
     );
     for b in &mut bases {
         b.aci = aci;