PanicException: byte index is not a char boundary — count_tokens on multibyte-dense input >~11KB

`count_tokens` splits long inputs at byte offsets without checking UTF-8 character boundaries. Any text past roughly 11KB that is dense in 3-byte codepoints raises a pyo3 `PanicException` ("byte index is not a char boundary") instead of returning a count.

Hit in production on two real inputs: long CJK prose, and a vision-model caption that rendered a masked password field as a ~4000-char `•` run.

**Versions:** tokie 0.0.10 / chonkie-core 0.10.2 / chonkie 1.6.8 (latest at time of writing), reproduced via chonkie's public API:

```python
from chonkie import RecursiveChunker

chunker = RecursiveChunker.from_recipe(
"markdown", lang="en",
tokenizer="Qwen/Qwen3-Embedding-8B",
chunk_size=384,
)
chunker.chunk("数据库连接配置说明文档" * 400) # panics
chunker.chunk("•" * 4000) # panics
```

Both raise `pyo3_runtime.PanicException` from `count_tokens`. Note the panic is a `BaseException`, not an `Exception`, so a bare `except Exception` in a pipeline won't catch it — without a specific guard it costs the whole document.

**Expected:** the same counts/boundaries HF `tokenizers` produces. We verified the HF backend counts the same vocab without the byte-slicing issue and produces identical boundaries on benign text (that's our current workaround: catch the panic and retry once on an HF-`tokenizers`-backed chunker). Presumably the fix is snapping split offsets to `char_indices` boundaries before slicing.

Happy to provide more repro detail if useful.