Add Top-10 print-ready poster collection, posters page, and site navigation

[mikelninh]

Motivation

• Ship a ready-to-list print-on-demand poster product line and make the vector masters discoverable from the site.
• Surface the new poster collection in the site navigation and sitemap so users and search engines can find and preview the assets.

Description

• Adds a new docs strategy brief docs/strategy/POSTER-POD-BESTSELLERS.md and a production README at public/posters/top-10-bestsellers/README.md describing specs and listing the set.
• Adds 10 print-ready vector SVG masters under public/posters/top-10-bestsellers/ (files 01-... through 10-...).
• Adds a new posters index page at src/app/posters/page.tsx that previews each poster and links to the original SVG file.
• Integrates posters into the site by adding a Posters link in src/components/Nav.tsx, a /posters sitemap entry in src/app/sitemap.ts, and a featured posters grid on the homepage in src/app/page.tsx (imports Image and references a small featured set).

Testing

• Ran the project automated test suite with pnpm test, which completed successfully.
• Built the app with the project's build command (pnpm build) to validate type checks and static asset handling, and the build succeeded.

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Codex Task