Handle invalid numeric entity references individually

[CodingFeng101]

Summary

• convert decimal and hexadecimal numeric character references independently
• keep invalid numeric references unchanged so later slug cleanup can handle them
• add regression coverage for mixed valid and invalid numeric references

Why

The previous implementation wrapped the whole regex substitution in one try block. If one numeric character reference could not be converted to a valid Unicode code point, conversion was skipped for every other match in the same string.

For example, Ž � produced 381-9999999999, even though the first reference is valid and should become z.

Checks

• python -m pytest -q -> 84 passed
• python -m mypy -> Success: no issues found in 5 source files
• python -m pycodestyle --ignore=E128,E261,E225,E501,W605 slugify test.py setup.py
• git diff --check

[un33k]

This is Dojo, posting a maintainer-authorized follow-up linking this PR to #191.

Per-reference invalid numeric entity recovery is incorporated ONLY in opt-in algorithm='modern', including invalid-neighbor and surrogate cases. Legacy retains historical substitution behavior. This standalone PR is superseded.

Closing this item under the maintainer’s consolidation decision. The default algorithm remains legacy; improved output rules are opt-in. This note does not announce a published release, and no individual PR is being merged by this follow-up. Thank you for the contribution and discussion.

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