M4: right-to-left and mixed-direction text (Unicode bidi) (#34)

Text runs get bidi embedding levels from the Unicode bidi algorithm
(unicode-bidi, MIT/Apache-2.0). Right-to-left runs are shaped right to
left, kept in logical order for line breaking and placed in visual order
per line (rule L2, trailing spaces at the paragraph level). Glyphs and
lines know their direction, so the caret, clicks and selection
highlights follow right-to-left runs. Text without right-to-left
characters skips the analysis.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Cargo.toml

@@ -43,6 +43,7 @@ usvg = "0.48"
 krilla = "0.8"
 skrifa = "0.47"
 harfrust = "0.13"
+unicode-bidi = "0.3"
 linesweeper = "0.4"
 zip = { version = "2", default-features = false, features = ["deflate"] }
 sha2 = "0.10"