Constellation, Spacedust, Slingshot, UFOs: atproto crates and services for microcosm

Bikeshed: many-to-many small follow-up #49

Merged microcosm.blue
targeting main from m2m-bikeshed
Merged opened by and assigned to microcosm.blue
targeting main from m2m-bikeshed

the derives came in from base64 in tangled#7, whic was removed.

the cursor is a composite, but we might have other composite cursors that are different -- renaming this one to reflect its specific purpose.

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