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/app/mod.rs

@@ -1504,6 +1504,11 @@ pub enum Message {
     PropAttrCommit(String),
     /// Toggle the inline color picker dropdown open/closed.
     PropColorPickerToggle,
+    /// Toggle the MTEXT background-colour picker dropdown open/closed.
+    PropBgColorPickerToggle,
+    /// User picked an MTEXT background fill colour — sets `background_color`
+    /// and switches the background mode to Fill so it shows immediately.
+    PropBgColorChanged(AcadColor),
     /// Collapse the inline color picker dropdown. Fired when another
     /// properties-panel dropdown (a combo_box) opens, so at most one panel
     /// dropdown is open at a time and they can't overlap. (#235)