Digest compilation now requires system inspection.

Specifically, @{digest} now sources @{inspect} and hence sets the zeshy-specific
unique identifier ${ZESHY_SYSTEM_ID}. The current digest file and configuration
directory then depend on such identifier, rendering both digests and
configurations system-specific. Broadly, this renders zeshy itself considerably
more robust in the face of cross-system migration and should theoretically allow
a single top-level "~/.zeshy" directory to be shared between multiple different
machines and users without modification.

In other words, win condition.