Function build_root_command_info 

fn build_root_command_info(
    command: &Command,
    registry: &CommandRegistry,
) -> Option<CommandInfo>

Build a synthetic CommandInfo for the root/default invocation.

Returns None when the root requires a subcommand or has no root-only args (i.e. nothing to invoke without a subcommand). The returned command has an empty subcommands list; named subcommands remain top-level siblings.

Registry lookup uses the empty path (&[]), so a binary can pin a stable id and capabilities for its default invocation (e.g. anvil.start).