Implement Sorted Leaderboard with Persistent Player Index

Description:

The get_leaderboard() function currently returns an empty vector and serves as a placeholder. This needs to be fully implemented to support the game's social competition features.

Current Behavior:

get_leaderboard() accepts a limit: u32 parameter but always returns an empty Vec regardless of registered players or their XP. Expected Behavior: The function should return the top N players (where N = limit) sorted by XP in descending order.

Requirements:

• Maintain a registry of all player addresses whenever a new player registers
• The leaderboard query should respect the limit parameter
• Players should be ranked by their current XP value
• The implementation must work within Soroban's storage constraints — note that iterating all storage keys directly is not supported on Soroban; a separate index structure must be maintained

Acceptance Criteria:

• get_leaderboard(5) returns the top 5 players by XP
• Results are sorted highest XP first
• A new player registration updates the player index
• A test covering leaderboard ordering is included