add Bidi_Mirroring_Glyph property

[9drbxz8jnt-hash]

Summary

• exposes Bidi_Mirroring_Glyph as uucode.get(.bidi_mirroring_glyph, cp) ?u21
• derives the new field from the existing deterministic ucd/BidiMirroring.txt parser while preserving the existing bidi_mirroring field
• adds regression coverage for mirrored punctuation/brackets/math symbols plus default-null cases

Verification

• zig build
• zig build test

Note

This targets zig-0.15 because Ghostty v1.3.1 is pinned to Zig 0.15.2. Upstream main currently requires Zig 0.16.0, so this branch is the compatible path for existing Ghostty consumers.