feat(terminal): add Unicode precision — CJK ambiguous width, emoji ZWJ, ligatures
- Add grapheme_width() with emoji ZWJ sequence detection (width 2)
and comprehensive is_cjk_ambiguous() per UAX #11
- Add TerminalFontOptions::{ligatures, cjk_ambiguous_wide} options
- Add is_emoji_presentation() covering major emoji Unicode blocks
- Update default font stack with CJK and emoji fallback families
- Add --terminal-font-ligatures CSS variable via font-variant-ligatures
- Add 8 unit tests for grapheme_width covering ASCII, CJK, emoji,
ZWJ sequences, combining chars, and variation selectors
- Mark P8 complete in roadmap
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