✨ NEW: Add section reference plugin (section_ref)

[chrisjsewell]

What

Adds a new section_ref inline plugin that captures section-sign references — §1, §1.1, §2.3.4 — the syntax LLMs commonly use to refer to numbered headings in documents.

See §1, §1.1 and §2.3.4 for details.

Each reference becomes a dedicated section_ref token:

• content: the matched text (e.g. "§1.1")
• markup: "§"
• meta["numbers"]: the parsed section number as a list of ints (e.g. [1, 1]), so consumers don't re-parse the dotted string (a list rather than a tuple so Token.as_dict()/JSON round-trips are type-stable; leading zeros are normalized, with the original text preserved in content)

so downstream renderers (notably MyST-Parser) can resolve them to heading cross-references. The default rendering is a neutral <span class="section-ref">§1.1</span>, overridable via md.add_render_rule("section_ref", ...).

Initial use case

executablebooks/MyST-Parser#1170 (draft) consumes this plugin as a new section_ref MyST extension, resolving the references to internal links targeting the correspondingly numbered heading of the document. That PR is blocked on this one being merged and released (it needs mdit-py-plugins >= 0.7).

Matching rules

• § must be immediately followed by ASCII digits ([0-9], never unicode digits), optionally in further dot-separated groups — § 1 and bare § stay literal text
• trailing punctuation is not consumed: see §1. captures §1
• a directly-following ASCII letter rejects the match (§1a, §1.2b), while non-ASCII letters do not block it (见§3章 captures §3)
• standard CommonMark backslash-escaping applies: \§1 stays literal
• inline code, code fences, autolinked URLs, and link destinations/titles are unaffected

Implementation notes

§ is not one of markdown-it's default text-rule terminator characters, so a plain inline rule would never fire mid-paragraph. The plugin therefore uses md.inline.add_terminator_char("§"), the same mechanism as the gfm_autolink plugin, and raises a RuntimeError on markdown-it-py < 4.1.0 (package dependency range unchanged).

Testing

• 24 new tests: token-contract test, version-guard test, and 22 rendering fixtures (tests/fixtures/section_ref.md) covering emphasis/link/code/heading contexts, boundary and escaping edge cases
• full suite passes (507 tests); ruff + mypy --strict clean under both markdown-it-py 3.x and 4.x
• adversarially probed against a MyST-style plugin stack (attrs, myst_role, dollarmath, gfm_autolink, footnote, deflist, anchors), GFM autolink URLs containing §, link-label posMax slices, the typographer, and cross-instance state leakage

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.79%. Comparing base (d11bdaf) to head (6a4040d).
⚠️ Report is 14 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
- Coverage   92.80%   92.79%   -0.02%     
==========================================
  Files          31       40       +9     
  Lines        1835     2289     +454     
==========================================
+ Hits         1703     2124     +421     
- Misses        132      165      +33     

Flag	Coverage Δ
pytests	92.79% <100.00%> (-0.02%)	⬇️

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