d2mcpp

Authoring conventions, design principles, and file formats for the d2mcpp (D2X) Modern C++ tutorial project. Use this whenever you add or edit a C++ language-feature lesson — a book chapter, a dslings exercise, a reference solution, or when registering them in SUMMARY/xmake/changelog. Trigger it for any request like "add a chapter for X", "write a dslings exercise for fold expressions", "add the constexpr lesson", "fill the cpp14 section", or anything touching book/src, book/en/src, dslings/, or solutions/. Even small edits should follow these conventions so the bilingual + book/code/checker artifacts stay in sync.