Write a SpacetimeDB backend module in C# that defines a table with optional (nullable) fields and a reducer that inserts rows with and without optional values.

TABLES
- Player
  - Struct: Player
  - Fields:
    - Id: ulong (primary key, AutoInc)
    - Name: string
    - Nickname: string? (nullable)
    - HighScore: uint? (nullable)

REDUCERS
- CreatePlayer(ctx, string name, string? nickname, uint? highScore)
  - Insert a Player row, passing through the nullable fields as-is
  - Use Id: 0 for AutoInc
