Add an English-specific frontpage template so the default homepage no longer falls back to the Chinese frontpage, and clear homepage image backgrounds in the frontpage wrapper CSS.
Inject a homepage-only language switch into the mdBook frontpage wrapper so the English homepage links to the Chinese homepage and the Chinese homepage links back to the English homepage.
Move the English root site to mdBook, keep the Chinese site as a sub-book, and update CI/deploy to publish .mdbook outputs to docs/ and docs/cn/. Also add regression coverage for placeholder skipping, publish-tree assembly, and shared resource setup.
