Characters being drawned with incorrect offset on lines with emoji characters

### Checklist

- [ ] Using latest version on the [App Store](http://itunes.apple.com/app/id1156707581)
- [x] Read the [docs](http://blink.sh/docs/)
- [x] Searched for [existing GitHub issues](https://github.com/blinksh/blink/issues)

### Configuration

Blink version: b82e66b4fc3fa59bbb2d5e2e90f2747ae442dff4 (built from source)
iOS version: 17.5.1
Starship config:

```toml
add_newline = false

[line_break]
disabled = true
```

### Describe the bug

When custom shell prompt contains emoji characters:

1. Character offset, e.g. whitespace disappear before `in`.
2. Ghost characters, e.g. the dual arrow.
3. Prompt character (the arrow) missing or can be deleted.

Expected behavior is a consistent prompt.


Start by reproducing the rendering problems with the reported Starship configuration, emoji characters, Blink build b82e66b4fc3fa59bbb2d5e2e90f2747ae442dff4, and iOS 17.5.1. Done means prompt spacing, arrow characters, and deletion behavior remain consistent on emoji-containing lines.

Written by the indexing model from the issue text.