Improve Blog navigation and Astryx Markdown rendering

[walishcs]

What changed

• default the Blog directory to Year while retaining the First tag switch
• render Blog bodies through Astryx Markdown with theme-native headings, lists, links, code, quotes, and tables
• remove a duplicated leading article title and clarify Keystatic heading guidance
• upgrade Astryx core, CLI, and all four supported themes from 0.3.0 to 0.4.3
• publish the existing bkuw Blog post (draft: false)

Why

Astryx reset styles removed native Markdoc list markers and heading spacing, causing missing bullets, oversized markers, and cramped article typography. The official Astryx Markdown renderer restores consistent themed formatting.

Validation

• npm run check
• npm test (48 tests)
• npm run format:check
• npm run build with Pagefind indexing
• npm run astryx -- doctor
• desktop browser verification of the current Chinese article, including H1/H2 hierarchy, disc and decimal lists, and Year directory default
• confirmed production output excludes old/ and Keystatic routes