Write a SpacetimeDB backend module in C# that defines a table, a non-reducer helper function, and a reducer that uses the helper.

TABLE
- Result
  - Struct: Result
  - Fields:
    - Id: int (primary key)
    - Sum: int

HELPERS
- Add: given a:int and b:int, returns int

REDUCERS
- ComputeSum: given id:int, a:int, b:int, insert exactly this row into Result
  - (Id=id, Sum=Add(a, b))
