feat(mtext): character-level text selection on the preview

The rendered MText preview is now interactive: drag across glyphs to select
a character range and apply toolbar formatting (bold, italic, underline,
overline, strike, case, alignment) to exactly those characters.

The text layout gains an opt-in pass that emits one world-space box per
visible character carrying its reading-order index; the editor maps those
boxes into the preview frame, hit-tests pointer drags against them, and
highlights the selection. Formatting splices the matching inline codes into
the raw value at the selected byte range (mapped via a code-aware visible-
character scanner). With no preview selection the Edit-box path is unchanged.

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

src/app/update.rs

@@ -3546,6 +3546,20 @@ impl OpenCADStudio {
                 self.rebuild_mtext_preview();
                 Task::none()
             }
+            Message::MTextSelStart(off) => {
+                if let Some(ed) = self.mtext_editor.as_mut() {
+                    ed.sel_anchor = off;
+                    ed.sel = Some((off, off));
+                }
+                Task::none()
+            }
+            Message::MTextSelTo(off) => {
+                if let Some(ed) = self.mtext_editor.as_mut() {
+                    let a = ed.sel_anchor;
+                    ed.sel = Some((a.min(off), a.max(off)));
+                }
+                Task::none()
+            }
             Message::MTextOk => {
                 self.mtext_commit();
                 Task::none()