Write a SpacetimeDB backend module in C# that defines two tables and a reducer that looks up a row by primary-key index and writes a projection to another table.

TABLES
- User
  - Struct: User
  - Fields:
    - Id: int (primary key)
    - Name: string
    - Age: int
    - Active: bool

- Result
  - Struct: Result
  - Fields:
    - Id: int (primary key)
    - Name: string

REDUCERS
- LookupUserName: given id:int, find the User row with that Id using the index and insert (Id, Name) into Result.
