Write a SpacetimeDB backend module in C# that defines a Player table with scores, a Leaderboard result table, and a reducer that finds the top N players by score (descending) and writes them to the result table.

TABLES
- Player
  - Struct: Player
  - Fields:
    - Id: ulong (primary key, AutoInc)
    - Name: string
    - Score: ulong

- Leaderboard
  - Struct: LeaderboardEntry
  - Fields:
    - Rank: uint (primary key)
    - PlayerName: string
    - Score: ulong

REDUCERS
- BuildLeaderboard(ctx, uint limit)
  - Collect all players into a list, sort by Score descending
  - Take the first `limit` entries
  - Insert each into Leaderboard with Rank starting at 1
