Function populate 

pub fn populate(registry: &mut CommandRegistry) -> ExBuiltins

Register every built-in ex-command into registry and return the ids callers need by field (see ExBuiltins).

Call once per registry, at boot, next to builtins::populate. Every spec’s apply only packages its parsed args into an Effect; the host performs the side effect. Subsystem crates (multibuffer, compilation, ai, …) register their own ex-commands at install time rather than here.