Every run used to write all three files (en.md, ru.md, json) unconditionally.
Default is now English Markdown only, sourced from output.formats in
config.yaml; --format/--ru/--json pick exactly what a run writes, and any
explicit --format replaces the default rather than adding to it (all/none
supported). write_report() moved from a fixed 3-positional-arg signature to
a {format: text} mapping so writes are conditional instead of always-three.
The en/ru Markdown cross-link header is now only set when both languages are
actually being written.
