Implement slugify from the specification

textkit.slugify in src/textkit/slugify.py is declared, documented and

tested, but its body raises NotImplementedError. Write it.

Contract

slugify(text: str, separator: str = "-") -> str builds a URL slug:

1. The result is lowercase.
2. ASCII letters (a-z, A-Z) and digits (0-9) are kept.
3. Every other character — including spaces, punctuation and non-ASCII letters

such as ż or é — acts as a separator. There is no transliteration.

1. A run of one or more separator characters collapses to a single separator.
2. The result never starts or ends with separator.
3. An input with no letters or digits yields an empty string.
4. separator is a single character and is inserted verbatim.

Examples

Applying slugify to its own result must not change it again.

Boundaries

• Implement it in src/textkit/slugify.py; the public API of the textkit

package stays as it is.

• Do not modify the tests and do not add third-party dependencies — the standard

library is enough.

• The repository's own test suite (pytest) has to stay green, so

textkit.casefold_word must keep working.