availability

Whether this bolt can be written to on this runtime.

Mirrors Loom.availability(): a consumer learns that a volume is read-only, or a directory could not be created, by asking rather than by crashing. A bolt reporting BoltAvailability.Available must accept an append.

This answers "can this bolt write now", not "does this runtime have a bolt". A backend with no home on a target is simply not compiled there — reaching for it is a compile error, which is a better answer than a runtime one, and it is why there is no wasmJs backend reporting BoltAvailability.Unavailable for the browser's missing filesystem. InMemoryBolt is what a browser gets; see its KDoc for what that does and does not buy you.