fix: Make heading IDs CSS-safe while preserving Unicode text

[josbeir]

Summary

This PR hardens heading ID normalization to avoid invalid CSS selectors, while preserving Unicode heading text (for example Japanese/Cyrillic headings).

Changes

1. Updated HeadingIdTracker::normalizeId() in HeadingIdTracker.php

• Strip #
• Trim whitespace
• Convert whitespace runs to -
• Replace invalid selector characters with - using Unicode-aware regex:
  • /[^\p{L}\p{N}_-]+/u
• Collapse repeated dashes
• Trim leading/trailing dashes
• Fallback to heading when result is empty

2. Updated tests

• HeadingIdTrackerTest.php
  • Added coverage for:
    • special-character input from inline-code-like heading text
    • dash collapsing
    • Unicode preservation (日本語の見出し)
    • empty-result fallback (### -> heading)
• TableOfContentsExtensionTest.php
  • Updated inline math TOC ID expectation:
    • Equation-E=mc^2 -> Equation-E-mc-2

Why

Some consumers (for example HTMX scroll restoration via querySelector) require selector-safe IDs. The previous logic could emit unsafe IDs for punctuation-heavy headings, and the first revision over-sanitized non-ASCII headings. This change keeps IDs safe and internationalization-friendly.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.55%. Comparing base (2dde660) to head (08f0a3f).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #92   +/-   ##
=========================================
  Coverage     93.54%   93.55%           
- Complexity     2312     2315    +3     
=========================================
  Files            79       79           
  Lines          6107     6113    +6     
=========================================
+ Hits           5713     5719    +6     
  Misses          394      394           

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[Copilot]

Pull request overview

This PR updates heading ID normalization to produce CSS-selector-safe IDs while preserving non-ASCII/Unicode heading text, and aligns related TOC expectations in tests.

Changes:

• Harden HeadingIdTracker::normalizeId() by trimming, whitespace-to-dash normalization, Unicode-aware character filtering, dash collapsing, and empty-result fallback.
• Add/adjust PHPUnit coverage for punctuation-heavy headings, dash collapsing, Unicode headings, and empty-result fallback.
• Update TOC test expectation for inline-math-derived heading IDs.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
src/Renderer/HeadingIdTracker.php	Implements stricter, Unicode-aware heading ID normalization intended to be safe for CSS selectors / querySelector.
tests/TestCase/Renderer/HeadingIdTrackerTest.php	Adds new normalization assertions (special chars, dash collapsing, Unicode preservation, empty fallback).
tests/TestCase/Extension/TableOfContentsExtensionTest.php	Updates TOC ID expectation for inline math to match new normalization rules.

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