Normalize configured schemes and fix XHTML footnotes

[dereuromark]

Summary

• normalize configured scheme lists in SafeMode and LinkPolicy so mixed-case configuration behaves correctly
• render the footnotes separator as <hr /> in XHTML mode
• add regression tests for mixed-case scheme configuration and XHTML footnote output

Verification

• vendor/bin/phpunit tests/TestCase/SafeModeTest.php tests/TestCase/LinkPolicyTest.php tests/TestCase/DjotConverterTest.php
• vendor/bin/phpstan analyse src --no-progress
• vendor/bin/phpunit

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.82%. Comparing base (e5b8a4b) to head (2b958a3).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #162      +/-   ##
============================================
+ Coverage     93.78%   93.82%   +0.03%     
- Complexity     3041     3044       +3     
============================================
  Files            93       93              
  Lines          7833     7833              
============================================
+ Hits           7346     7349       +3     
+ Misses          487      484       -3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.