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/mod.rs

@@ -740,6 +740,10 @@ pub enum Message {
     /// Switch the editor body between raw code input (`false`) and the
     /// rendered preview (`true`).
     MTextShowPreview(bool),
+    /// Begin a preview text selection at the given visible-character offset.
+    MTextSelStart(usize),
+    /// Extend the preview selection to the given visible-character offset.
+    MTextSelTo(usize),
     /// Commit the editor: create or update the MText entity.
     MTextOk,
     /// Discard the editor without creating / changing the entity.