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

style/tailwind.css

@@ -638,6 +638,7 @@
     );
     font-size: var(--terminal-font-size, 13px);
     line-height: var(--terminal-line-height, 18px);
+    font-variant-ligatures: var(--terminal-font-ligatures, none);
     white-space: pre;
     letter-spacing: 0;
     scrollbar-gutter: stable;
@@ -689,6 +690,7 @@
     );
     font-size: var(--terminal-font-size, 13px);
     line-height: var(--terminal-line-height, 18px);
+    font-variant-ligatures: var(--terminal-font-ligatures, none);
     white-space: pre-wrap;
     word-break: break-word;
   }