style(blog): full-width blog posts on desktop

[konjoinfinity]

Summary

Blog reading pages felt cramped on wide desktop screens: the site-wide .md-grid { max-width: 1200px; } rule left large empty margins on either side of the three-column layout (author/metadata sidebar, article content, table of contents) once the viewport exceeded ~1200px.

• Individual blog posts render a .md-post marker inside their metadata sidebar (author, publish date, etc.) — a marker that's absent on regular docs pages and (as far as I can tell) on the blog listing/archive/category pages, since those show post excerpts without the per-post sidebar.
• Added a body:has(.md-post) .md-grid rule, scoped to the min-width: 76.25em desktop breakpoint (Material's own breakpoint for when both sidebars are visible), that removes the 1200px cap and lets the grid use the full viewport width (with small side padding) specifically on blog post pages.
• Mobile layout and all non-blog docs pages are untouched.

Type of change

• Bug fix
• New feature
• Performance improvement
• Documentation update
• Refactor / cleanup

Checklist

• No hardcoded absolute paths
• No model weights, eval output files, or log files staged for commit
• Changes are scoped to one logical concern (blog post desktop width)
• pytest tests/ passes locally — N/A, docs-only change
• ruff check squish/ tests/ reports no errors — N/A, no Python changed
• Performance-sensitive changes include a before/after squish bench run — N/A

Verified locally with mkdocs build + Playwright screenshots at three viewports:

• 2000px wide: blog post now spans the full viewport width
• Regular docs page (Installation) at the same 2000px width: unaffected, still uses the normal centered narrow layout
• 420px (mobile) blog post: unaffected, identical to before

Related issues

Follow-up to #215–#219.

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