fix: avoid char-boundary panic in NBReader::try_read

[SAY-5]

The internal buffer stores raw input bytes as latin-1 chars, so a byte >= 0x80 becomes a two-byte char and drain(..1) could land off a char boundary and panic. Drain the first char by its UTF-8 length instead.

Closes #32

[epage]

Our contrib guide asks for a particular commit structure. See also https://epage.github.io/dev/pr-style/#c-test

[SAY-5]

Split into a test commit that shows the panic on multi-byte input followed by the fix, per the contrib guide.

[SAY-5]

Restructured per the c-test guide: the test commit now documents the panic with should_panic, and the fix commit drops should_panic and asserts the decoded chars. Pushed.