17 new tests covering remaining scenarios:
- md5_composite: manual calc, dirs excluded, compression-independent,
used in resolve_local_file
- storage tiers: external, user_provided, embedded, fetch_large_file
with cache hit and hash rejection
- data_directories: suppress cross-ref gaps when matched, show gaps
when unmatched
- shared groups: _shared.yml includes injection, empty group safe
- YAML inheritance: systems inherited, verification_mode override
- platform grouping: identical merged, different base_dest separated
