v0.7.0-zeus

perf: defer VSS schema version check to first use

VssStore::new previously blocked on a network round-trip to determine
the VSS schema version during construction. When the VSS server is slow
or unreachable, this caused the Zeus app's 15-second build timeout to
fire, silently falling back to local-only storage without VSS backup.

Move the schema version determination into a lazily-initialized
OnceCell, so VssStore::new completes instantly with no network calls.
The schema version is resolved on the first actual read/write/list
operation instead.

With DualStore, this means the VSS round-trip happens on a background
thread (during bulk-sync or fire-and-forget writes), never on the
critical startup path. Users always get a proper DualStore with both
local and remote persistence.