Nested plugin groups with cascading disable for cordis-rs.
A group is an entry with a group array in the config file (see
cordis-include). At runtime the loader starts each group entry as a
[Group] fiber and starts the child entries beneath that fiber's
context: disposing the group fiber cascades to the whole subtree, and a
disabled flag anywhere on the ancestor chain keeps the subtree from
starting at all (the include tree's enabled() walk).
The plugin itself is deliberately a no-op nesting marker — upstream
cordis' plugin-group is similarly tiny. All orchestration lives in
cordis-loader, which registers [GROUP_NAME] in its builtin registry.