Type assertions require an interface type, not a concrete struct pointer.
Changed all integration tests from 'mgr := pkg.New()' to
'var mgr snack.Manager = pkg.New()' so capability checks like
mgr.(snack.VersionQuerier) compile correctly.
Implements the dnf sub-package with Manager, VersionQuerier, Holder,
Cleaner, FileOwner, RepoManager, KeyManager, Grouper, and NameNormalizer
interfaces.
Implements the rpm sub-package with Manager, FileOwner, and
NameNormalizer interfaces.
Both follow the existing pattern: exported methods on struct delegate to
unexported functions, _linux.go for real implementations, _other.go with
build-tag stubs, embedded snack.Locker for mutating operations, and
compile-time interface checks.
Includes parser tests for all output formats.
