Fix DictSupplementaryFileContainer: refcount never incremented, delete_file never frees memory

[zrgt]

Fixes #495

Changes

• _assign_unique_name: increment _store_refcount[sha] when creating a new _name_map entry (first loop branch) — without this, count stays at 0 and delete_file's == 0 check never triggers
• rename_file: decrement _store_refcount[file_hash] when removing the old name entry from _name_map, consistent with delete_file behavior
• Add test_supplementary_file_container_refcount — verifies backing store is freed only after the last reference is deleted