Add Djot markup support (#13)

* Add Djot markup support

Adds support for the Djot markup language using php-collective/djot:

- DjotInterface and DjotMarkup converter class
- DjotHelper view helper for templates
- DjotView for rendering .djot template files directly

Features:
- XSS protection via safeMode (enabled by default)
- XHTML-compatible output option
- Profile-based feature restriction (full, article, comment, minimal)
- Variable substitution in DjotView templates using {{varName}} syntax

Djot is a modern markup language created by John MacFarlane (author of
CommonMark/Pandoc), offering cleaner syntax and more consistent behavior
than Markdown.

Fix code quality and security issues (Issues 3, 4, 5, 7, 8) (#12)

* Fix code quality and security issues

VideoFilter Improvements:
- Fix uninitialized variable bug (initialize $t = null before conditional use)
- Improve regex validation (check preg_match() !== 1 for proper error handling)
- Replace empty() with direct boolean check for cleaner code
- Document incomplete TODO with explanation of timestamp limitation

Security Enhancements:
- Add class validation before dynamic instantiation in BbcodeHelper
- Add class validation before dynamic instantiation in MarkdownHelper
- Validate classes exist and implement required interfaces
- Prevent potential code injection via invalid converter classes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Fix phpstan

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Co-authored-by: Claude <noreply@anthropic.com>

Update CI

Fix up plugin name.

Merge remote-tracking branch 'origin/master' into cake5

# Conflicts:
#	phpunit.xml.dist

Fix up phpunit config

Allow v2 of commonmark

Merge branch 'master' into cake4

meta files.

Update .travis.yml