v4 de-prose: src/v4/extdeps/languages/cpp.dag — 93.4%→<20% strict Practice-9 (modeling-discipline.md); FOLLOW-UP fresh PR on main-state (#3267 merged un-de-prosed, in-PR fix impossible); COMMENT-ONLY, carrier model byte-identical (v2 compile 0 diagnostics unchanged); cpp_abi.dag already compliant — - #3291