fix: improve blog heading semantics

[corwinm]

Summary

• use each blog article title as its single h1
• promote article section headings from h3 to h2 for a correct heading hierarchy
• preserve the existing 24px section-heading presentation
• rename the article return link from Back to overview to Back to Blog
• add regression coverage that discovers every blog.*.mdx article and enforces the heading contract

Test plan

• TDD RED: all three article-heading cases and the return-label case initially failed
• TDD GREEN: focused semantics tests pass 4/4
• pnpm lint — 0 warnings/errors
• pnpm typecheck
• pnpm test — 40/40 passing
• pnpm build
• git diff --check
• production SSR smoke test: all three article routes contain one h1, h2 sections, and Back to Blog
• browser accessibility-tree verification on /blog/arashi: one level-1 title followed by level-2 sections
• browser computed-style verification: section headings remain 24px
• independent review caught an initial h1 → h3 hierarchy skip; fixed by promoting sections to h2, then re-reviewed successfully with no security or logic concerns

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Project	Deployment	Actions	Updated (UTC)
corwinm-remix	Ready	Preview, Comment	Jul 13, 2026 8:55pm