Root `|2` block scalar: js-yaml and libyaml-family disagree silently

At document level the YAML grammar puts the parent indentation at -1, so --- |1\n a\n means " a\n" (content at column 0). js-yaml 4/5 and goccy follow that. PyYAML, libyaml, ruamel, yaml (npm), go-yaml, serde_yaml, yaml-rust2 and saphyr read "a\n" (they count from 0). Writers are split the same way: serde_yaml/go-yaml write |2-/|4- meaning n=0, and js-yaml 5/yaml (npm) write |3-/|1- meaning n=-1. The result is silent corruption in both directions: serde_yaml's dump of " a\nb" comes back from js-yaml as " a\n b", js-yaml 5's dump of "\n a" comes back from 15 other parsers as "\na", and go-yaml reads js-yaml's " a\nb" as "a". The yaml-test-suite has no case with content after an explicit indicator at document level, so nobody's CI catches it. Fix on your side: never emit a multi-line string as the document root. Wrap it in a mapping, or force double-quoted style.