[vim/vim] helptoc: FuzzySearch() highlights matched characters at wrong column for multibyte text (Issue #21056)

VimWei created an issue (vim/vim#21056)

### Steps to reproduce

1. Create a markdown file with a multibyte heading, e.g. a single line # 中文标题
2. :set filetype=markdown
3. :colorscheme default — the fuzzy-match highlight uses the IncSearch group; some colorschemes make it indistinguishable from the surrounding text, so a default colorscheme makes the effect visible
4. :packadd helptoc | HelpToc
5. Press / and type 标题

**Actual behaviour**

The highlight lands on the wrong column — it overlaps a neighboring character and splits a multibyte character (only the first byte of the 3-byte UTF-8 char is highlighted).

### Expected behaviour

Each matched character 标 and 题 is highlighted (via the help-fuzzy-toc text property, IncSearch).

**Cause**

matchfuzzypos() returns match positions as character indices (:h matchfuzzypos(): "list of character positions ... You can use byteidx() to convert a character position to a byte position"). But text property col/length are in bytes (:h prop_add(), :h popup-props: "counted in bytes").

In runtime/pack/dist/opt/helptoc/autoload/helptoc.vim, FuzzySearch():

```viml
props: pos[i]->copy()->map((_, col: number) => ({
    col: col + 1,
    length: 1,
    type: 'help-fuzzy-toc',
}))
```
col + 1 treats a character index as a byte column, and length: 1 is one byte instead of the byte length of the matched character. This is only correct for pure-ASCII text (where character index equals byte index), which is why it goes unnoticed in English help files.

**Suggested fix**

Convert character indices to byte offsets with byteidx() and use each matched character's byte length:

```viml
props: pos[i]->copy()->map((_, cp: number) => {
    const col = byteidx(match.text, cp) + 1
    const len = byteidx(match.text, cp + 1) - byteidx(match.text, cp)
    return {col: col, length: len, type: 'help-fuzzy-toc'}
})
```

### Version of Vim

9.2.0907

### Environment

- Windows 10 
- gvim 9.2.0907  (also present in 9.1)

### Logs and stack traces

```shell

```

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