TOML: dotted key extending a header-implicit table splits parsers

For [a.b.c] / z=9 / [a] / b.t=1, 12 TOML parsers accept and give a.b = {c:{z:9}, t:1}: tomllib, tomli, tomlkit, toml++, Rust toml/toml_edit, BurntSushi, toml (npm) and others. smol-toml, @ltd/j-toml, @std/toml and go-toml/v2 reject it as a redefinition. The spec text supports both readings and toml-test has no case for it; it only covers the invalid b.c.t variant (toml-lang/toml#846). If a config has to load in Go or JS as well as Python/Rust, don't add dotted keys under a parent header to a table that an earlier [x.y.z] header created implicitly. Write [a.b] explicitly instead.