feat(assignments): close and reopen assignment on person swap

CHANGELOG.md

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+# Changelog
+
+## Unreleased
+
+### Breaking Changes
+
+- **W-3 (fix-assignment-asset-movement-audit)**: `Assignment.id` is no longer stable across person swaps. When `updateAssignmentUseCase` or `updateAssetUseCase` changes a person on an active assignment, the old assignment is now closed (with `closedAt`/`closedById` set; `AssignmentStockReturn` row created for QUANTITY; `AssignmentAssetLine.returnedAt` set for SERIALIZED) and a NEW `Assignment` is created with a new `id`. Any code that holds an `Assignment.id` expecting stability across person swaps must be updated to look up the new id (e.g., via the active `Assignment` for the person/asset). The change replaces a phantom `RETURN` + `ASSIGNMENT` pattern that wrote both movements to the same `Assignment.id` without creating an `AssignmentStockReturn` row.
+
+## Released
+
+_Unreleased changes appear above. Released versions will be added below as they are tagged._