review(#1629): one direction mark, no dead RTL paragraph path, tests follow the new layout

- One visible-named LRM constant (ttf_glyph) instead of two invisible
  literals. Stroke and SDF text skip it, so a right-to-left script the
  shaper does not handle no longer gets a 6-unit gap per mark; vertical
  MTEXT does not insert it at all.
- Shaped cells keep a cluster the shaper merged into the leading mark.
- The always-false RTL paragraph flag, its indent/anchor branches and the
  unused reorder_line_atoms are gone; its test now covers reorder_rtl_runs.
- Underline, overline and strike follow each drawn run of a mixed word.
- Justify/distribute count only the gaps the measured width holds.
- The two RTL alignment tests assert the new left-to-right behaviour;
  glyph_boxes keeps its own doc comment.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/entities/text.rs

@@ -705,9 +705,13 @@ mod tests {
         let doc = codec::CadDocument::default();
         let placement = text_run_placement_at_scale(&t, &doc, 1.0);
 
-        // For Arabic text with default HA::Left, origin should be shifted left
-        // so the right edge of the text sits at the insertion point (X = 10.0).
-        assert!(placement.origin[0] < 10.0, "Arabic text origin must be to the left of insertion point: origin_x={}", placement.origin[0]);
+        // Left alignment does not depend on the script: Arabic text starts
+        // at the insertion point like any other.
+        assert!(
+            (placement.origin[0] - 10.0).abs() < 1e-9,
+            "Left-aligned Arabic text starts at the insertion point: origin_x={}",
+            placement.origin[0]
+        );
     }
 }