fix(mtext): support arabic cursive shaping and bidirectional rtl text layout

- Enable fallback font metrics for HarfBuzz shaping in TTF glyph resolver

- Implement Unicode Bidirectional Algorithm (UBA) paragraph base direction detection

- Anchor right-to-left paragraphs to right container boundaries by default

- Reorder line atoms with base direction level to preserve neutral punctuation attachment

- Right-align RTL single-line TEXT and handle editor caret placement on RTL glyphs

src/app/mtext_editor.rs

@@ -640,16 +640,16 @@ fn vertical_caret_target(
     let anchor = boxes
         .iter()
         .find(|item| item.vis == caret)
-        .map(|item| (item.xmin, item.ymin, item.ymax))
+        .map(|item| ((if item.is_rtl { item.xmax } else { item.xmin }), item.ymin, item.ymax))
         .or_else(|| {
             caret.checked_sub(1).and_then(|previous| {
                 boxes
                     .iter()
                     .find(|item| item.vis == previous)
-                    .map(|item| (item.xmax, item.ymin, item.ymax))
+                    .map(|item| ((if item.is_rtl { item.xmin } else { item.xmax }), item.ymin, item.ymax))
             })
         })
-        .or_else(|| boxes.first().map(|item| (item.xmin, item.ymin, item.ymax)));
+        .or_else(|| boxes.first().map(|item| ((if item.is_rtl { item.xmax } else { item.xmin }), item.ymin, item.ymax)));
     let Some((anchor_x, anchor_y, anchor_top)) = anchor else {
         return caret.min(visible_count);
     };

src/app/view/overlay.rs

@@ -141,10 +141,18 @@ impl MTextPreview {
             let d = dy * 1000.0 + dx; // prefer the correct line first
             if d < best_d {
                 best_d = d;
-                best = b.vis;
-                // After the glyph centre → caret sits after this char.
-                if wx > (b.xmin + b.xmax) * 0.5 {
-                    best = b.vis + 1;
+                if b.is_rtl {
+                    if wx < (b.xmin + b.xmax) * 0.5 {
+                        best = b.vis + 1;
+                    } else {
+                        best = b.vis;
+                    }
+                } else {
+                    best = b.vis;
+                    // After the glyph centre → caret sits after this char.
+                    if wx > (b.xmin + b.xmax) * 0.5 {
+                        best = b.vis + 1;
+                    }
                 }
             }
         }
@@ -280,14 +288,21 @@ impl iced::widget::canvas::Program<Message> for MTextPreview {
             );
         } else if collapsed {
             let bar = if let Some(b) = self.boxes.iter().find(|b| b.vis == self.caret) {
-                Some((b.xmin, b.ymin, b.ymax)) // left edge of the caret's glyph
+                let cx = if b.is_rtl { b.xmax } else { b.xmin };
+                Some((cx, b.ymin, b.ymax))
             } else if self.caret > 0 {
                 self.boxes
                     .iter()
                     .find(|b| b.vis == self.caret - 1)
-                    .map(|b| (b.xmax, b.ymin, b.ymax)) // after the last glyph
+                    .map(|b| {
+                        let cx = if b.is_rtl { b.xmin } else { b.xmax };
+                        (cx, b.ymin, b.ymax)
+                    })
             } else {
-                self.boxes.first().map(|b| (b.xmin, b.ymin, b.ymax))
+                self.boxes.first().map(|b| {
+                    let cx = if b.is_rtl { b.xmax } else { b.xmin };
+                    (cx, b.ymin, b.ymax)
+                })
             };
             if let Some((cx, y0, y1)) = bar {
                 let p0 = map(cx, y0);