Phase 4: Prepare expectation data structures and graph utilities

[3vilM33pl3]

User Story\nAs a designer of the expectation system, I need foundational data structures (graph relationships between buckets, expectation definitions) so that future commands only focus on business logic.\n\n## Acceptance Criteria\n- Define expectation-related structs/enums in the data layer with serde + validation, even if not yet persisted.\n- Provide utility functions for loading/saving expectation metadata (e.g., from .b/expectations) with tests.\n- Document how expectation graphs will be represented to unblock follow-up stories.