Harden cross-sheet entity merging and relationship inference

[bensonlee5]

Context

Cross-sheet entity merging and relationship inference exist now, but they are still heuristic and fragile.

Missing

• Repeated monthly sheets can still merge or split incorrectly when naming shifts.
• Lookup/reference tables and fact tables need more reliable foreign-key style linking.
• Stable entity IDs need to survive naming noise better.

Scope

• Improve cross-sheet entity merging with stronger name, field-set, and provenance signals.
• Harden lookup/reference relationship inference.
• Reduce false merges and false relationship edges in ambiguous workbooks.

Acceptance

• Goldens cover repeated monthly entities, lookup/reference tables, and fact-table linking.
• Relationship edges are confidence-scored and provenance-backed.
• Canonical entity IDs remain stable across repeated sheets with minor naming variation.