feature: SSD prompt cache

[ianscrivener]

See https://github.com/orgs/mflux-community/discussions/605#discussioncomment-18074162

[fxd0h]

Unparking note: the freeze this was filed under (#588) was about landing 0.19.0, and with 0.19.1 out too it's over. The marker can come off whenever someone picks this up.