feat(learn): Expose Deep Dive subsection

[Xuepoo]

Summary

• serialize Zola subsections alongside direct section pages
• expose the canonical Deep Dive title and description as a first-class Learn card
• keep the Deep Dive section as the single source of its sixteen article entries
• run Bun regression tests in the CI verify job

Closes #47

Validation

• bun test — 14/14, 423 assertions; includes a real Zola build and 2/2 Deep Dive payload assertions
• bun run check
• bun run lint:md — 578 Markdown files, 0 issues
• bun run build — 562 pages / 22 sections
• Playwright: Deep Dive card is visible on /learn/, navigates to /learn/deep-dive/, and exposes the Overview article
• independent incremental review: no findings; PX-0142 resolved

Scope

The primary checkout has unrelated local Deep Dive localization work. This PR is based directly on origin/main and contains only the Learn section template, its regression test, and the CI test gate.