fix(bytes-str): Fix BytesString split_off char boundary

[kdy1]

Summary

• Add a char-boundary assertion before BytesString::split_off mutates its backing buffer.
• Document the panic behavior and correct the prefix/suffix wording for split_off.
• Add regression coverage for valid Unicode splitting and invalid mid-codepoint splitting.

Why

BytesString::split_off could split inside a UTF-8 code point, breaking the internal UTF-8 invariant and allowing safe APIs like as_str to produce invalid str data.

Fixes #90.

Validation

• cargo fmt --check -p bytes-str
• cargo test -p bytes-str

[djc]

(The deny failure should go away if you upgrade to version 2 of the action.)

[djc]

Are you planning to publish a new release with this?

[kdy1]

Yeap, I'll fix auto publishing of this repository once I get back to my PC.

I thought f9ff0d8 would publish a new version.

Thanks for the ping

[kdy1]

Done 😄