v4.3.1

Fixes packaged Markdown preview assets introduced in v4.3.0.

• Load bundled highlight.js, KaTeX, and Mermaid assets through Sublime Text's resource API so .sublime-package installs work correctly.
• Preserve traversal protection for the reserved asset route.
• Add packaged-resource regression coverage.

Verified with all 38 automated tests and a real packaged install in Sublime Text and Brave. Fixes #8.