Custom completions should support subcommands after flags

Related problem

I would like to be able to represent subcommands after flags when working with custom completions:
Often the top level command will have flags that it's necessary to put before the subcommand.

$ cmd --flag subcmd --subflag foo

My naive way of representing this command would be:

export extern cmd [
  --flag
  ...args
]

export extern "cmd subcmd" [
  --subflag
  bar: string
]

But then the completions bottom out after cmd --flag, and I don't get the completions for subcmd

Describe the solution you'd like

It would be great if cmd <options> subcmd just worked, so that after the flags, it "passes through" to the extern "cmd subcmd" definition and uses those completions.

Describe alternatives you've considered

No response

Additional context and details

No response

Start with the custom completion definitions shown in the issue, especially how extern cmd and extern "cmd subcmd" are selected after flags. Reproduce cmd --flag subcmd --subflag foo and trace completion handling for ...args; done means subcommand completions remain available after top-level flags and the existing extern "cmd subcmd" definition supplies them.