Heading anchors now keep the same characters as GitHub: letters, marks,
decimal and letter numbers, connector punctuation and alphabetic
symbols, with ASCII spaces turned into `-`. The rule follows
[github-slugger](https://github.com/Flet/github-slugger), whose fixtures
are generated from real GitHub renders.
- Other whitespace, including newlines in multi-line headings, is
dropped instead of becoming `-`, so `1<em space>ab` gets `#1ab`
- Combining marks are kept, so `हिन्दी` no longer becomes `हनद`
- Other numbers like `²` and `①` are dropped
Fixes https://github.com/go-gitea/gitea/issues/27754
This PR was written with Claude Code.
---------
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: silverwind <me@silverwind.io>
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