Add RTL/BiDi support for Arabic, Hebrew, and other RTL scripts

[Tirador1]

Summary

Fixes #725.

• Adds a post-render Unicode BiDi reordering pass that correctly displays Arabic, Hebrew, Syriac, and other RTL scripts in terminals that don't implement BiDi (e.g., Windows Terminal)
• Reverses characters within RTL runs and mirrors bracket pairs, while preserving LTR text (English, code) unchanged
• Zero overhead for pure-LTR content via fast-path containsRTL check
• No new dependencies — uses golang.org/x/text/unicode/bidi which is already a transitive dependency

Before

Arabic text appears with reversed characters and wrong word order:

ترامس ماش    (should be: شام سمارت)

After

Arabic text displays correctly when read right-to-left:

شام سمارت    ✓

Changes

File	Description
bidi.go	New — ANSI-aware BiDi reordering: strips escape codes, applies Unicode BiDi algorithm per line, reverses RTL runs with bracket mirroring, reconstructs styled output
main.go	1 line — calls bidiReorder(out) after glamour renders

Test plan

• Pure Arabic headings and paragraphs render correctly
• Mixed Arabic/English text preserves LTR segments in correct position
• Pure English markdown is completely unaffected (byte-identical)
• Code blocks remain LTR
• ANSI styling (colors, bold, italic) preserved across reordering
• Unit tests pass for pure RTL, mixed LTR/RTL, and edge cases