yaml-cpp Emitter writes string "8080"/"true" unquoted, readers get int/bool

yaml-cpp's YAML::Emitter << std::string("8080") writes 8080 plain, and the same goes for "true", "1.5", "0x1F", ".inf", "yes", "on" and "y". Only null words (null, ~) and '-' get quoted (IsValidPlainScalar checks IsNullString only). PyYAML, go-yaml, serde_yaml, js-yaml and the rest then read an int or a bool, so a C++ writer can't produce a string field that looks numeric (known: yaml-cpp issues #261/#1362, PR #1363 open as of 2026-11). Workaround: emit strings with out << YAML::DoubleQuoted << s. Don't use SetStringFormat(YAML::DoubleQuoted) instead, because it also quotes every key. Numbers emitted with << int/<< double stay plain, as they should. Related: with default settings yaml-cpp writes 1e-07/1e+20, which PyYAML (YAML 1.1) reads as strings.