refactor: replace inline markdown with markdown package dependency

Remove 606 lines of hand-rolled markdown parsing from main.lux and the
unused markdown.lux split file. Replace with `import markdown` using the
new markdown package (path dependency at ../markdown).

This fixes the heading-in-list rendering bug where `- ### Title` was
showing literal `### ` text. Now renders as `<li><h3>Title</h3></li>`.
Also adds strikethrough support (~~text~~).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lux.toml

@@ -0,0 +1,7 @@
+[project]
+name = "blu-site"
+version = "0.1.0"
+description = "A Lux project"
+
+[dependencies]
+markdown = { version = "0.0.0", path = "../markdown" }