Constant BLOB_TOMBSTONE_GRACE 

pub const BLOB_TOMBSTONE_GRACE: Duration;

The default convergence window a host gets if it configures none: how long a deleted blob is kept after its tombstone is written, before a GC pass reclaims it. The host overrides it on the coven builder; the writer’s tombstone collection evaluates whatever grace it is handed against the tombstone’s deleted_at.

A device offline for less than the grace is never stranded by a deletion — when it reconnects it pulls the removal of the row that referenced the blob, and the blob is still present until then. The window is human-scale (days, not the sub-second commit window the snapshot sweep’s grace covers) because the device it protects is a person’s offline laptop or phone, not a concurrent writer mid-publish.