Handle other read-only participants

See merge request !4

They get an empty immutable directory which ends up as a directory literal cap
which we need to avoid trying to parse as a directory ssk cap.

adjust implementation state

See merge request !3

Implement basic read-only support for magic-folders

Closes #2

See merge request !2

It was pedagogical

I can't figure out how to make it pass reliably.

SIGNED-OFF-BY: SHAE

what is the word like "additive" but instead of "things are only added" it
means "things are only corrupted"?

* un-hard-code configuration in the example app
* replace a bunch of partial pattern matches with nice big exhaustive cases
* update some comments about which errors we are losing and where

Co-authored-by: Shae <shae@scannedinavian.com>