feat(mtext): structured editor with selection, copy, and visible bold

Rewrite the in-app MTEXT editor to run on acadrust's structured
MTextDocument model end-to-end: load via parse_mtext, edit a flat Cell
model, serialize via to_mtext_string. Retires the hand-rolled format-code
string handling, the parallel visible_spans parser, the composed_value
prefix, and the Edit-raw-code tab.

Selection and copy:
- double-click selects the word, triple-click selects all (click-counting)
- Ctrl+A selects all text
- Ctrl+C copies the selected text to the clipboard

Formatting:
- per-selection Bold/Italic/Underline/Overline/Strike/Case/Alignment
- Bold renders as a visibly thicker SDF pen, keyed (family, char, bold)
  through the whole text pipeline; the resolved style font name is stamped
  so the \f...|b1; code survives serialization
- per-selection font and color changes
- editor color dropdown reuses the Properties panel color selector

Global toolbar defaults (font/color/oblique/width/tracking) fold onto the
document as real span properties instead of a leading code prefix; a
trailing empty paragraph is preserved so Enter at the end keeps its line.

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

src/app/commands/blocks.rs

@@ -54,6 +54,18 @@ impl OpenCADStudio {
             }
 
             "COPYCLIP" => {
+                // MText editor open: Ctrl+C copies its selected text, not the
+                // drawing's entities.
+                if self.mtext_editor.as_ref().is_some_and(|e| e.show_preview) {
+                    return match self.mtext_selected_text() {
+                        Some(text) => {
+                            self.command_line
+                                .push_info("Copied selected text to clipboard.");
+                            Some(iced::clipboard::write(text))
+                        }
+                        None => Some(Task::none()),
+                    };
+                }
                 let handles: Vec<_> = self.tabs[i]
                     .scene
                     .selected_entities()