feat: support Unicode grapheme clusters in node names

[at-manos]

Changes

• Replaces ASCII string operations with grapheme-aware slicing/access to handle multi-byte characters, such as CJK, accents, and emojis.

Tests

• README examples
• Emojis
• Chinese characters
• Latin accented characters

Preview

```graph
😀 - 笑;
😢 - 哭;
笑 < 夭,竹;
哭 < 口,犬;
犬 < 大;
夭 < 大, 丿
\```

```graph
é - à;
à - ü;
ü - ö;
ö - ñ;
ñ - á;
á - ô;
ô - í;
í - é;
é - ü;
ñ - í;
\```

```graph
N1.N7 - 网;
网 > A;
\```