Fix reverting leaderboard and replace O(n^2) sorting with bounded top-K

[Xaxxoo]

Summary

• Fix empty leaderboard revert: Return an empty array when registeredPlayers.length == 0 or topN == 0 to prevent Solidity 0.8 checked arithmetic underflow on total - 1.
• Replace O(n^2) bubble sort with bounded top-K insertion sort: The new algorithm is O(total * topN), which is effectively O(total) for constant topN values (e.g., 10 or 100). This eliminates the quadratic cost of copying and sorting the entire player set on every call.
• Define stable tie-breaking: Players with equal totalScore are ordered by registration order (earlier registration first).
• Add getLeaderboardPage(limit, offset): Pagination support for off-chain UIs that page through the full leaderboard.
• Comprehensive test coverage: Tests for 0 players, topN=0, single player, tied scores, large-N with small topN, oversized topN clamping, zero-score players, and pagination edge cases.

Closes #8

Test plan

• Verify getLeaderboard(10) returns empty array with zero registered players (no revert)
• Verify getLeaderboard(0) returns empty array with registered players
• Verify single-player leaderboard returns correct entry
• Verify topN > totalPlayers is clamped correctly
• Verify tied scores maintain registration-order tie-breaking
• Verify top-K selection from 10 players requesting top 3
• Verify pagination via getLeaderboardPage across multiple pages
• Run npx hardhat test to execute all tests

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