[feature] Simplify and generalize the Relationship enum #519

The current Relationship enum in overture-schema-system has 7 domain-specific members (BELONGS_TO, BOUNDARY_OF, CAPITAL_OF, CAPITALLED_BY, CHILD_OF,
CONNECTS_TO, DESCENDANT_OF). This design has a few problems:

1. It conflates structural category with domain semantics. CAPITAL_OF and CAPITALLED_BY are specific to the divisions theme but live in the shared system package.
2. It doesn't scale. Every new kind of relationship across any theme requires adding a new enum member to the core system package.
3. It mixes coupling strengths. There's no way to distinguish a lifecycle-dependent structural relationship (building part → building) from a loose peer reference (segment → connector) at the type level.

Proposed change

Replace the 7 members with 4 abstract semantic types that describe coupling strength rather than domain-specific meaning:

• COMPOSITION — structural whole-part, lifecycle dependent
• AGGREGATION — grouping without lifecycle dependency
• HIERARCHY — organizational/classificatory nesting
• ASSOCIATION — peer-level reference

Domain-specific meaning (e.g., "capital_of", "boundary_of") moves to an optional role field on the Reference annotation, allowing themes to express their semantics without modifying the core enum.

Motivation

• Aligns with the relationship model proposed in Pydantic relationships #511
• Makes the system package stable as new themes and relationship patterns are added
• Provides a clear decision framework (diamond model) for schema authors choosing a relationship type