Cause: `visual_gaps` mapped a gap at a run boundary to the gap before the later run's first item,
which for a run nested inside another (a Chinese word in a Hebrew phrase in Chinese text, made
with embedding marks) is a gap inside the embedding: one boundary took it, another got none, and
the line came up a gap short of the box's edge.
Fix: each gap is reordered with the line as a glyph of no width at the lower of its two
neighbours' levels (rule N1 for a space between them), through the same `visual_order`: items and
gaps keep alternating and each gap lands once at the boundary it belongs to. Test row with explicit
embedding marks (red before: "char 8 moved 2 (want 1)").
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>