Implements Manifest and Entry structs with YAML tags, New() constructor,
Load(path) for reading, and Save(path) with atomic write (temp file +
rename). Tests cover round-trip serialization, atomic save cleanup,
entry type invariants, nonexistent file error, and empty manifest.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
