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 <noreply@anthropic.com>

src/entities/attribute.rs

@@ -208,6 +208,7 @@ fn build_attr_truck(input: AttrTextInputs<'_>, document: &acadrust::CadDocument)
             v_anchor,
             line_spacing_factor: 1.0,
             vertical_text: false,
+            want_glyph_boxes: false,
         });
         let _ = input.line_count;
         let _ = input.is_multiline;