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/table.rs

@@ -267,6 +267,7 @@ impl TruckConvertible for Table {
                     v_anchor,
                     line_spacing_factor: 1.0,
                     vertical_text: false,
+                    want_glyph_boxes: false,
                 });
                 // Flatten TextStroke groups into the table's Lines buffer.
                 // Per-run inline `\C` / `\c` colour is dropped here because the